Where do you get a dark soul gem in Oblivion?
Dark Soul Gems need to be made at Dark Fissure (east from the shrine of Vaermina), using grand soul gems. A complete guide can be found here
to get a black soul gem you need to find a necromancers alter. when you find one make sure you have a empty soul gem.(GRAND)soul gems are better.note.you can do this as many time as you like.place your empty grand soul gem in to the alter and cast a soul trap spell. there must be a purple light shining down on the alter for it to work. for xbox 360 .ps3 and computer
To recharge enchanted items (such as weapons, armour, etc) you need to use Soul Gems, but only if there is a Soul trapped in the Soul Gem, otherwise it won't work. A Soul Gem with no Soul is practically useless unless you trap a Soul in it. I believe you can find Soul Gems by either buying them or finding them in treasure chests.
They are found on necromancers. There are two in Falcar's room in the Mages Guild, but they are used for a recommendation quest, and they count as stolen. You can get around the fact they are taken from you by taking one and leaving the other, giving thw one you have to Deetsan, and then taking the other one. Or, you can make them yourself by doing the Mages Guild quests until you get to…
Black soul gems sometimes appear randomly in the inventory of higher level necromancers. Otherwise, you can make your own black soul gems out of grand soul gems if you go to a Necromancer altar (there are four of them: Dark Fissure, Fort Istirus, Fort Linchal and Wendelbek) on a day when the purple light is shining on it (once every eight days). You put the Grand Soul Gem in the altar, cast Soul Trap and…