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
http://www.uesp.net/wiki/Oblivion:Black_Soul_Gem
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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
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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 the quest Necromancer's Moon. This quest sends you to a cave named Dark Fissure. There is an altar outside Dark Fissure. Every eight days, the Shade of the Revenant occurs - a beam of light shines on the altar. Check the UESP wiki to find out more about when that occurs. When the Shade of the Revenant is happening, place an empty grand soul gem in the altar. Cast a Soul Trap spell on the altar. Open the altar - the grand soul gem was transformed into a black soul gem.
You have to go to some cave(can't remember what it's called but you go there in the mages guild quest) put a soul gem on this altar outside the cave at night on a certain day of the week, then use a soul trap spell on the altar when a light is shining on the altar
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 the Gem is now black.
There are also Black Soul Gems involved in the Cheydinhall Recommendation Quest for the Mages' Guild, but they don't really work, unfortunately.