I looted a Black Soul Gem filled with a Grand Soul from a spell casting forsworn at Karthspire Camp.

I later entered Red Eagle Redoubt where my Dwarven Bow of Scorching became empty. I used my newly found Black Soul Gem (with a Grand Soul) to recharge it and my Ench lvled up. I then used my Dwarven Bow of Binding on the next forsworn I saw, and was told I do not have a soul gem large enough. I checked my inventory and my black soul gem had vanished. I reloaded, checked my inventory and sure enough I had a black soul gem filled with a grand soul. I then used the gem on my now not yet fully depleted bow of scorching, had my enchanting lvl up, but again my black soul gem had vanished. Have I encountered a bug, or do black soul gems get consumed by recharging a weapon? I thought for sure all my other soul gems remained with me when I used them to recharge weapons, are black gems different?

All soul gems are consumed upon use except for the unique one.

Good to know I guess. I am surprised I have never run out of them then! Is there a finite number of them in the game, or do they respawn in certain places and/or shops?

Only one type of soul gem is refillable but it's a spoiler so I wont say what it is. It has to do with a daedric prince however.

You will often find them in dungeons populated with mages of ill repute.

They do respawn both in stashes shelves etc, and merchant inventories.

Oh you can get more black soul gems, just raid the places where Necromancers hang out. They will all respawn eventually and their caves/dungeons will restock.

You can buy some from Falion I think. Also some dude at the college sells them over priced.

You can mine soul gems in Blackreach

Forsworn have them pretty frequently at mid-high levels as well.

A black soul gem is just a soul gem filled with a human soul. You are confusing a black soul gem with Azshara's Star which can be used as a reusable black soul gem.

Black Soul Gems can be filled with any type of soul, not just human.

I believe black soul gems can only be filled with human souls. Grand soul gems can be filled with any soul except human.

No, you have to be careful, Black Soulgems will happily eat a rabbit or something if it's the only one in your inventory. There's a bug or feature where you can drop them from your inventory and they will lose the soul inside, but it won't work (apparently) if the soul is from a damn Mudcrab. Just that one, it's driving me insane!

Two. But I won't spoil it for you either... :hubbahubba:

Spoilers about the star...

Azura's star only works with non-human souls. The black soul gem version is if you choose to turn it into the Black Star.