As far as I've seen (playing a Guardian and a Thief both based on some Blind) I can answer you:
1) An attack is a single strike or a single projectile or something like this, something that can miss and that triggers "attack effects".
2) Hundred blades inflicts 8 attacks, so the first one misses for Blind purpose, while others will go on target, unless the Blind source comes from a renewable source, like a field.
3) A meteor shower actually throws lots of attacks, notice how you could, with some luck, pass it unharmed if you manage to avoid the single meteors, so the first one hitting a target should miss, ending the blind effect (must try with my Elementalist, now that I think about it), while a frost or poison field is not really an attack itself, it doesn't make attacks, you trigger their effect by walking into them, so they are unaffected from Blind.
4) The Thief sample follows the same rule for condition fields, if it's the field "activelly" trying to hurt you (like a meteor shower, again) it is affected, otherwise it works normally, notice that the Blind condition doesn't prevent the attack to start, it prevents the attack to hit, so attacks which don't follow a hit/miss rule are not affected from Blind. Being the field normally created, combo skills will also work.
Again, this answer is based on my experience being sometimes the inflictor of Blind, and sometimes the one who get blinded. Hope this helps.
EDIT: For pulse AoE the Blind condition negates the first pulse for every foe on the AoE, but other effects and subsequent pulses will work as intended.
EDIT2: Of course, if the AoE is compose of a single hit, it will miss completely due to the Blind condition.
Bleeds
Bleed effects stack with intensity, meaning the more stacks you have the more damage it does every time it 'ticks'. The duration of each bleed is individually tracked; You can apply a 20 second bleed and then three 5 second bleeds all running at once, and you'll have 4 stacks for that 5 second period, and it will fall back to 1 stack after that until the long stack runs out.
Superior Sigil of Earth
The Superior Sigil of Earth has a 60% chance on critical to 'proc' and apply a bleed stack for 5 seconds. It has a 2 second internal cooldown, which means it can only proc once every 2 seconds. It has a chance to proc on every attack or ability you use.
Ignoring other bleed producing sources, assuming you crit 100% of the time, and you get lucky and proc every crit, you'll have a maximum of 5 stacks of bleeding on your target from auto attacks with this Sigil. Factoring in your ability use will push the number up, but in practice you'll need a very high crit rate to consistently see around 5 stacks.
Dual Wielding
Dual Wielding rules for sigil stacking are complex. For sigils that proc effects, they stack multiplicatively, they share the same internal cooldown, and they can trigger the effect with the same chance no matter what hand is attacking.
If you're dual wielding pistols with a superior sigil of earth, with a 60% proc rate, you have a 1-(1-.6)*(1-.6) = .84 = 84%
chance to proc the sigil on each critical strike.
Sharpshooter
Sharpshooter has a 30% chance on critical to proc and apply a bleed stack for 3 seconds. Like all traits of this nature, it can proc independently of your sigils.
If you have a Superior Sigil of Earth and Sharpshooter, its possible to proc two stacks of bleed (one 3 second stack and one 5 second stack) on a critical strike, if you get lucky.
With a high crit rate, you can expect to see around 8-10 stacks with both sharpshooter and two Sigils of Earth.
Best Answer
From playing with a dual dagger elementalist,
I believe that sigils which proc 'on critical hit' are tied to the weapons skills, so you're correct in assuming that the life stealing will only proc when using the first three skills.Correction: According to this thread: http://www.guildwars2guru.com/topic/43566-weapon-sigil-questions/ Critical Hit sigils should fire regardless on the skill used, even utility skills, so as long as the weapon is equipped, the effects should be active.
I haven't used the 'on critical hit' sigils too much, as I tend to rely on the 'on weapon swap' sigils, so I could be wrong.