[RPG] Monsters to Scale: Rules to determine or estimate the weight of monsters

I see nothing there that implies it is a barrier to vision, just an active and utter absence of illumination.

You are quite right - there is nothing in the spell that says it blocks vision, just that the area is in Darkness.

However, a strict reading of normal darkness means you can't see through that (PHB p.183):

Darkness creates a heavily obscured area. Characters face darkness outdoors at night (even most moonlit nights), within the confines of an unlit dungeon or a subterranean vault, or in an area of magical darkness.

And a Heavily Obscured area is (PHB p.183):

A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature in a heavily obscured area effectively suffers from the blinded condition (see appendix A).

Which has been erratad as:

A heavily obscured area doesn’t blind you, but you are effectively blinded when you try to see something obscured by it.

So, darkness (magical or otherwise) creates a heavily obscured area. A heavily obscured area "blocks vision entirely".

Now, while it is clear what this means for "opaque fog, or dense foliage" is simple and straightforward - you can't see into this stuff and you can't see through it to stuff on the other side of it.

Applying this to darkness, however, seems to result in nonsense because, in the real world, darkness isn't a thing. In the real world darkness is the absence of light hitting your eyes from a certain direction. This can be because of an actual absence of light (underground) or because, even though the region is full of light none of it is coming your way (space). But this isn't the real world, is it?

So you have 3 options:

1. Darkness works just like it says in the book - you cannot see into it or through it. This would be really cool for a gothic horror campaign even though it would make navigating at night ridiculously hard.
2. Darkness (magical or not) works as it does in the real world - you can't see into it but you can see through it to illuminated areas beyond. I think this is what the rules intended even though they and the errata were poorly drafted.
3. Normal darkness works like 2. Magical darkness works like 1. There is no support for this in the Darkness spell description but this is how it worked in prior editions.

Its your world - make it fun.

I've noticed the same thing, that is people using the term "house rule" in contexts that seem to be outside the lines I consider them to cover.

GMs do a lot of things - they set down rules acknowledged to be always in effect, they make judgement calls when using existing rules ("Is my familiar an animal for purposes of thing X or does it not count because it's a 'magical beast'?"), and they create new content for the game (among others).

Generally, a difference is drawn between rules, even house rules, and rulings, and content creation. Let's investigate what some games explicitly say about it. Most comments about house rules come from D&D and its variants, one might speculate that it's because other games have a surrounding culture that is a lot less, uh... discriminating... about the exact pedigree of a specific bit of game. These quotes are from recent versions but they reflect the convention in Basic/1e/2e to my recollection and experience.

Pathfinder talks about house rules and rule arbitration as two different but related activities:

In addition to these roles, the Game Master might also fill a handful of others. Many groups maintain a set of house rules for their games, and the Game Master has the final say on particular interpretations and arbitrations of rules (though everyone in the group should be aware of any house rules beforehand). - Pathfinder Gamemastery Guide, p.8

Establish House Rules: If your house rules differ from the main rules, make sure everyone knows about it. Also, be sure to let your players know that this isn’t a sport, and that you reserve the right to bend or break the rules for the sake of the game from time to time, with the understanding that your intention isn’t to be unfair, but rather to make things more fun for the group as a whole. - Pathfinder Gamemastery Guide, p.76

4e strikes a stark contrast between rules arbitration and house rules:

Creating House Rules As Dungeon Master, you wear several hats: storyteller, rules arbiter, actor, adventure designer, and writer. Some DMs like to add a sixth hat to that stack: rules designer. - D&D Dungeon Masters Guide (4e) p.189

There the section on creating house rules calls out being rules arbiter as a separate, required activity, and then has a whole section on house rules as a discrete optional activity.

Same thing with 3.5e:

CHANGING THE RULES Beyond simply adjudicating, sometimes you are going to want to change things. That’s okay. However, changing the rules is a challenge for a DM with only a little experience. Altering the Way Things Work Every rule in the Player’s Handbook was written for a reason. That doesn’t mean you can’t change some rules for your own game. Perhaps your players don’t like the way initiative is determined, or you find that the rules for learning new spells are too limiting. Rules that you change for your own game are called house rules. Given the creativity of gamers, almost every campaign will, in time, develop its own house rules. - D&D Dungeon Masters Guide (3.5e) p.14

But it's not always so simple. I've heard people refer to GM content creation as "house ruling," which I always thought was just plain wrongheaded, but in OSRIC, I noticed that while they refer to house rules in the usual "rule mod" sense (e.g. removing demihuman level limits) they also refer to creating a new magic item not in the book as "house ruling" an item.

if the GM chooses to house-rule a magic item or spell which has the effect hold undead(...) - OSRIC p.237

This is very interesting because it can be seen to use an expanded definition of house ruling - it's still net new "written down" stuff but expands it past where most people do.

In a game, there's "the world as the characters experience it" and there's "the stuff written down in the books." In that sense, you can interpret any gap between the two - which consists of a) rulings and arbitration around existing rules, b) net-new rules or modifications to rules, and c) content created that's not in those rules (even though the rules allow for creation of content) as "house rules..." But in the end that's not super helpful because those three activities are very different and most games acknowledge them as different.

1. Rulings happen all the time - they have to. It's like the fundamentalist fallacy - that there is such a thing as a pure literal reading of the Bible. There's not, it's impossible, human language and each person's understanding of it is a variable filter. If you say you're using rules without a filter of rulings, no you're not, you just maybe don't know a lot about human cognition.

Rulings aren't always "for forever," they can be situational, though of course the characters (and the players) do tend to start interpreting the world in the light of rulings so they are as powerful in crafting the experience as new rules. I had a player who got grabbed by a choker, which grapples around the neck and the victim can't talk/spellcast. I let him use his two-handed weapon while grappled since the grapple was described as "around the neck." When it happened to him again later, I had to decide whether that ruling was really going to stand every time or whether it was situational.

2. House rules is generally meant, as is proven by game book quotes, to be a more codified and permanent set of larger scale changes to the game. "You should let new players know about house rules" doesn't mean "you should let them know about every rule interpretation ever conceived of by the group," that's silly. The general use of this term clearly implies "large chunk" rules - these rules are gone or significantly modified (e.g. no demihuman level limits), rules options off limits or whatever.

3. Content creation is content creation; lumping it in with the other two is really unhelpful as the processes are completely different. It's like saying you're modding Fallout 4 when you're using the settlement building system to build settlements - it's just misleading. "Home brew" is the more correct term to use to indicate self-generated content - e.g. "my homebrewed catgirl race."

So while I've also heard people use "house rules" to mean both rulings and content creation, I think that interpretation is provably fringe/"wrong" as much as any human usage of language can be considered to be "wrong".

Many fifth edition players are new to the game, and cargo-cult terms (same thing with RAW and RAI, we saw a lot of confused misuse of them in early 5e questions) without fully understanding them. I don't think it has much to do with 5e being more ruling-friendly except inasmuch as the topic comes up more than if you're playing a more legalistic game. House rules were much rarer in 3e/4e than in previous eds, so it's also possible that becoming less used to the term over the last 15 years has contributed to its misuse now that it's revived.