I found a ruin that always has a black soul gem and I can duplicate how ever many that I need. Are they worth it?

Black won't hold any larger soul than Grand. It's just a little easier to fill, as it will take a NPC soul (Dremora, bandit, marauder etc.) which can be useful if you're using a heavily enchanted weapon in that kind of dungeon, and need to re-fill often.

Or if you're low-level, and want to do an enchantment that needs a grand soul. Any bandit can fill a black soul gem with a grand soul.

It's quite wonderful for a roleplay. Trapping the soul of a _______, then use that mob's attack or defense characteristics on a weapon or piece of clothing/armor

or even better

One of my old assassins, would create black soul gems, then only trap a grand soul in it.... then create a one-time, custom killing weapon for the assassination at hand. You must dispose of the weapon to eliminate all evidence. You cannot sell it. Now get crackin.

Wonderful for roleplay indeed! Assassins can add in the pretend challenge that whoever contracts for a hit requires proof of the deed - in the form of the victim's soul delivered in a black gem.

Buffy is not an assassin, but when Peryite asked her to go rescue the souls of his followers, she wondered if she might need black soul gems to help safely bring them back. Hey, she's only a young elf and didn't know any better.

Or if you have the stones for it, you could go take on the Unidentified Mutton Uderfrykte Matron at Dive Rock. She always has a grand soul.

Ah, yes, the Unfrightening Madam. She's not so tough, either. She's ridiculously weak to fire. I took her out with a level 1 mage a while back. The alternate start mod I use has as one of its starting points the glacier right above Dive Rock. She doesn't climb well. You just have to keep your distance above her, and wear her down with fire, for a long time.