http://www.uesp.net/wiki/Oblivion:Enchanting
Black Soul Gems
Regular Soul Gems can only trap creatures, not NPCs (note that some creatures, like Dremora, are actually classed as NPCs). During the Arcane University Necromancer's Moon quest, you learn the Necromancy technique to turn a regular Soul Gem into a Black Soul Gem, which is capable of trapping NPC souls. That same information is provided here.
Once you have a filled Black Soul Gem, it is used exactly like a regular one. A Black Soul Gem can hold as much as a Grand Soul Gem (1600), but it is much easier to fill since any humanoid is 1600 units!
Things you need to make a Black Soul Gem:
empty Grand Soul Gem
method of casting Soul Trap
How to do it:
Go to a necromancer dungeon (see end of Notes for the list).
On the outside, look for a white altar draped in red cloth.
Rest until evening. Starting at midnight on one day of the week, a beam of light will shine down on the altar and persist for 24 hours. It will appear once every week. If you don't see the beam of light, you've arrived on the wrong day. Keep resting in 24-hour periods until it shows up.
Activate the altar like a container and place an empty Grand Soul Gem inside it.
Cast Soul Trap on the altar.
Retrieve your Black Soul Gem.
Notes:
Be sure to bring several Grand Soul Gems whenever you do the ritual, to save time. You must cast Soul Trap once for every Grand Soul Gem inside the altar.
During the Necromancer's Moon quest, you will be sent to Dark Fissure specifically. At 1 AM on Middas, a necromancer will arrive to create a Black Soul Gem - you must observe this event to advance the quest. Afterwards, you can steal his new Black Soul Gem from inside the altar, and use it to soul trap him. Sort of ironic justice. If you have more empty Black Soul Gems, enter the Dark Fissure and fill up on the necromancers within.
If you want to Soul Trap more humanoids, check out the Ayleid ruin called Garlas Agea - it's full of necromancers. It is located about midway between Kvatch and Anvil, down a hill to the south side of the road. It is a fun fight for a low-level character and can be profitable in terms of potions and books looted. However, Garlas Agea does not have an altar.
The hastily scrawled note that directs you to Dark Fissure lists four locations: Dark Fissure, Fort Istirus, Fort Linchal, and Wendelbek. There are altars at all four of these. Dark Fissure is southeast of Cheydenhal. Istirus is south of the road between Skingrad and Kvatch. Linchal is a bit northeast of Kvatch. Wendelbek is on the north shore of the Panther River.
Dus met een lege grand soul gem naar een Altaar in een Necromancer's cave gaan. 1 dag in de week schijnt er rood licht op. Dan doe je de Grand soul gem erin, cast je soultrap op het altaar... En voila: een black soul gem (zelf nog niet geprobeerd, en wordt ook uitgelegd in een quest voor de mages guild)