Find and Refluff a Class or Ability
The inflict line looks like a handy set if you just want 'damaging blood'. The Dread Necromancer (3.5) class gets an ability called Charnel Touch that does small amounts of negative energy damage, why not.
Pick something that you can fluff similarly, preferably with a spell list (spell lists are great, pick the stuff you can refluff as blood-related, take eschew materials, ignore the rest).
Play a Black Blade Magus and fluff the black blade as being made out of your blood.
Write A Class
It's really not hard to write a class. Keep in mind the basics (level x 2 = overall attack bonus, level x 5 = overall damage per round, level x 10 hp for melee, half those numbers for spellcasters, 3/4 those numbers for half-casters), and just invent some thematic abilities in the style you want. You want to be a melee blood-user, right? So this class should a) give you a magical weapon that scales with level (blood slash, or whatever), b) give you some blood-based means of defense that scales with level (blood armour? fast healing?), c) some wildcard ability(s) (blood teleport? sanguine fury? rip blood out of someone and save vs death? burning blood defense?) at appropriate levels d) something unique and out of combat, like blood healing or blood scrying or assimilating people's memories by drinking their blood or something.
Call it the Cursed Blood Scion and you're cookin' with gas.
Cursed Blood Scion
Cursed Blood Scion
1 Cursed Blood, Bloody Frenzy
2 Tainted Blood, Fearsome Bloody Body
3 Murderous Blood, Blood Armour
4 Ancient Prophecy, Taste Taint
5 Know Blood, Terrifying Mien
Cursed Blood - Your blood can be used as a weapon. Striking someone with your blood, or a weapon formed from your blood, does 1d8 + half your class level as an enhancement bonus + 1.5 your str. Attacking someone with your blood is an attack action, and you add half your class level as an enhancement bonus to that roll.
Bloody Frenzy - If a creature dies or is disabled by lethal damage within 5' of you, you can cover yourself in their blood and go into a frenzy. You immediately may strike out at another foe within range with your Cursed Blood attack. This ability triggers even if it isn't your turn and you didn't kill or disable the creature. The creature may be an ally or an enemy.
Tainted Blood - Half the damage from your Cursed Blood ability is now Negative Energy damage. In addition, creatures must make a fortitude save to avoid being Shaken for the duration of the encounter when struck by your Cursed Blood ability. Once they make this save, they do not have to make another for 24 hours.
Fearsome Bloody Body - Whenever you use your Bloody Frenzy ability, you may immediately make an intimidate check with a +5 bonus vs any enemy with a clear line of sight. Whenever you succeed on an intimidate check by 10 or more, that foe is Frightened instead of Shaken.
Murderous Blood - When loosed from your body, your blood does it's best to kill people. Whenever you suffer a blow that deals hp damage, your opponent takes an amount of damage equal to half your class level, half slashing, half acid. If you should be knocked unconscious or killed, your blood forms into a small ooze-like creature and attacks enemies to the best of it's ability (see: Minor Blood Ooze creature entry). Should you be revived, it immediately returns to your body.
Blood Armour - You can form your blood into a protective armour that covers your entire body. This is a Full-Round Action, and counts as Full Plate Armour with an enhancement bonus equal to half your class level. It also looks fantastically badass, and gives you a +2 morale bonus to intimidate checks. You are fatigued as normal for heavy armour if you wear your Blood Armour while running, traveling, or sleeping.
Ancient Prophecy - An ancient prophecy sleeps within your veins, doom hidden in the curse that writhes within your blood. This allows you to tell the future by how cursed and doomed you feel. Once per day, you may use Divination with a 90% success chance but only to determine the likelihood of bloodshed in any specific circumstance.
Taste Taint - If you taste someone's blood, you can tell if they are Evil, Vile, Cursed (as if by Bestow Cursed), a Black Blood Oracle, or otherwise 'cursed' or 'tainted'. No spell can disguise this, but all you can tell is they are 'tainted' or not - not which specific source of 'taint' it might be. Tasting spilled blood in combat is a move action if you are the one who spilled it - standard otherwise.
Know Blood - If someone whose blood you have tasted is within your class level x 5' of you, you know which square they are in, and which person it is. You never forget the taste of someone's blood.
Terrifying Mien - If you are covered in someone else's blood, you can make a Demoralize intimidate check as a move action, instead of a standard. If you instead perform it was a full round action, you can attempt to Demoralize every enemy within 30'.
Walk The Red Road(8th, paimon blade dance), The Blood Remembers(long distance travel through blood), Sanguine Frenzy(murder things), Call of Blood(make it hard for foes to retreat), Bloody Allure(intimidate as diplomacy), That Red, Red Wine(better living through blood ingestion), Cleanse and Purify(disease removal), Bloody Grin (intimidate as will save, 12th), Blood Shield, Birthed In Carnage, Hemophage, Swim the Red River, Smell Blood, Evil for Evil, Blood for Blood, Death is a Metaphor, Blood-Drenched Heritage, Your Blood is My Blood, Crimson Vampire Knight, Flow In A Million Veins, Walk A Thousand Battlefields
First, you can't make people care about your backstory if they aren't interested in that sort of thing. I know it is frustrating, but in RPGs, there are so many different things people want out of the game, and not everyone cares about the backstory. Heck, some don't even care about the story. If they are just looking for a dungeon crawl or murder hobo game, they might not be interested in the backstory of your character.
Second, share it with your DM/GM/StoryTeller. Your DM may, or may not, wish to tie events to characters' backstories.
Third, you could (out of game) look at tying your backstory in with another player's backstory, so at least to that other player there is something to be gained from those questions. Especially if you work with/through your DM/GM/StoryTeller – maybe you know the answer to one of the questions one of the other players is seeking.
Lastly, backstory informs roleplay. Maybe it isn't necessary to fish for people asking, or to beat people over the head with the backstory, instead just roleplay the character thinking about what they would do in each situation, and maybe backstory points will come up. "Oh, DM/GM/Storyteller, do I know anything about this symbol from my time studying while at the temple of ... ?" Etc.
No, there are not. Your character is obese if you say he is. That’s it. Done.
The description of a character is entirely up to the person playing that character.1 It is desired that mechanics “back up” description, so you should think about mechanics in terms of the desired description, but this does not require special rules. It’s a simple matter of keeping the description in mind while making all the various character-creation choices you are going to make.
There are a number of different ways you can do this, and which you choose depends entirely on how you imagine the character and how you imagine his or her obesity affects their life. So the way to handle this is to think about what your obese character might be like when you are choosing your various options.
All of the choices you make when creating a character can and should be viewed through the lens of “which choice here can back up the image of the character that I want to project?” Your character, though obese, is going to be adventuring: that has a lot of requirements on a person, from exploring, to traveling, to keeping safe, to killing monsters. Most adventurers are physically fit because adventuring is demanding. How does your character not get fit while adventuring? On the flip side, how do they do all the things adventuring requires without relying on physical fitness?
Note that it is possible to answer all of these questions within the normal framework of character creation. It is also possible to do so without hampering yourself: somehow or another, your character has figured out a solution to these questions that allows him or her to be an effective adventurer while neither having nor gaining physical fitness. You don’t need special penalties: just focus your character-creation resources in directions that reflect this reality.
For example, ability scores: a low Dexterity but high Constitution is a common way to model obesity. This isn’t a special bonus or penalty, since you are not supernaturally or even superhumanly obese. You are just on the high end for natural weights for your race; thus you are on the low end for typical Dexterity scores, and on the high end for typical Constitution scores (if you choose to model it this way). And Constitution is good for a lot of characters, while many characters don’t really need Dexterity. Someone who is obese isn’t likely going out for the things that rely on Dexterity, not when their life depends on them being able to do something effectively. Much better to rely on something else for safety.
For another another option, you may choose to avoid classes and feats that improve your speed. In fact, you can make choices that reward not moving very much. You may choose classes and feats that allow you to avoid significant physical activity: wizard with Heavily Armored, perhaps, so you can stand in place, protected, while casting spells.
And these are definitely not the only options. Every choice can be looked at from this perspective. Skills? Probably not Athletics. Items? Maybe something to provide good meals, or a mount to avoid the large amounts of walking that adventuring might require. Whatever.
So again, being obese is what you make of it. You don’t need special rules for it.
As for the feat in Book of Vile Darkness, that is specifically gorging yourself for the express purpose of disfiguring your body by acquiring grotesque obesity, all as an homage to some evil patron. That is why that is evil – and why it gives special bonuses. It’s ritualistic and profane. If you just are obese, either because you eat too much or because of some medical issue, that doesn’t make you evil and doesn’t require the feat (but also doesn’t come with the dark rewards that the feat does).