I read somewhere that black soul gems can be found in Necromancer hideouts, but I've cleared three of them already and there wasn't a single one.

It's only the bosses (the Adepts), and it's only a 10% chance that they will be carrying one. Also, you have to be above level 5.

You can make your own, if you have a Grand Soul Gem, and

I'm lvl 12.

Is that quest related or can I just walk in there and do it?

In any case, that's a lot of trouble just to start a quest. I'm glad the offering to shrine thing was done away with in Skyrim.

Both. The ability to do what glargg described is available to anyone who knows of it and using that ability does not break or hurt any quest at all. If you progress the Mages Guild questline however, you will learn about the ability.

I like both the offering and level requirements of Oblivion's Daedric quests because it adds more steps/length/complexity to things. At six years and counting, I'm in no hurry to 'finish' the game.

Yeah, I like "trouble" in my games.

I like that there are requirements for the Daedric quests, but the level requirement always felt artificial to me. If they're really looking for "a more powerful champion" it seems like http://oblivion.nexusmods.com/mods/42614 would be a better thing to check.

Anyway, the issue I have with the whole Black Soul gem thing is that the process is so strict that how would you roleplay a character (not me as a player) actually figuring out how to make one without doing the mages guild quest line?

I don't mind so much that it requires being a certain level, but it would be better if it wasn't so obvious as to the fact. Mostly not the journal entry having to specifically mention the level required. Although I don't consider it a big deal, perhaps it would be better if you could only get the quest if you are the level required. Which does make each quest require both that you have the item that is wanted, and you are the correct level, which could cause a problem, depending on how one plays the game.

It could be through experimentation, although that likely has limits. I don't know if it is possible to see the ritual being performed outside of the MG questline, which might not be a bad idea for a mod. Although it would certainly be the case where, even if a character knows quite a bit about magic, it could still be that they would have no idea as to how it works, even if they were able to see the ritual. Which might require the one performing the ritual to have a scroll that would detail the process, even if it was fragmented(such as a non-journal mini quest, having to find a few necromancer's performing the ritual, and at a certain point, you'd know enough so that you can do it yourself).

One possibility would be to imagine asking one of the followers at the shrine. "Black soul gems? What are they, and where am I supposed to get one?" To which the follower replies, "Oh, you just get yourself a Grand Soul Gem, and you go to..."

It's not possible to see the ritual except during the quest. I've had many characters witness the...visible signs...and trace them back... but then what? There's absolutely nothing to connect that with the idea of making black soul gems. The only remote hint we get is

So again it's unlikely that anyone will stumble across it unless they're running the mages guild quest line. But the process is only explicitly detailed in the note which we get during the quest.

I suppose it would be easy enough to make a mod that adds a small chance of finding a copy of the note on necromancers or maybe add it to certain dungeons. But that's a different solution to a different problem.

That's why your mod is essential to my game. I don't mind slight "troubles" like finding a black soul almost impossible at low levels, but I really find level requirements pointless, also because I use a mod that makes leveling much slower.