You can obtain Black Soul Gems in Oblivion through several methods: 1. Creating Black Soul Gems from Grand Soul Gems:

• Purchase or find Grand Soul Gems from alchemical or magic merchants, such as Aurelinwae at the Mystic Emporium in the Imperial Market District or the Mages Guild Halls.
• Take a Grand Soul Gem to one of four specific altars located at:
  • Dark Fissure (southeast of Cheydinhal by the Road River)
  • Fort Istirus (east of Kvatch)
  • Fort Linchal (northeast of Kvatch)
  • Wendelbek (by the Panther River)
• Wait for a "Shade of the Revenant" day, which occurs every 8 in-game days starting from day one, lasting 24 hours from midnight to midnight. During this time, a purple light appears over the altar.
• Place the Grand Soul Gem on the altar, then cast a Soul Trap spell on it. This will transform the Grand Soul Gem into an empty Black Soul Gem

2. Finding Black Soul Gems on Necromancers:

• Black Soul Gems can be found as rare loot on necromancer enemies, especially necromancer adepts and bosses in necromancer dungeons.
• Notable necromancer lairs include Dark Fissure, Fort Istirus, Fort Linchal, and Wendelbek, which also have altars.
• The drop rate is low, so clearing these areas repeatedly may yield Black Soul Gems

3. Other Notes:

• You can also start the Daedric quest for Vaermina, which requires a Black Soul Gem.
• Buying the Soul Trap spell (available from magic merchants) is essential for creating Black Soul Gems.
• Stealing Grand Soul Gems from display cases in the Chorrol Mages Guild is a common early-game method to get gems for conversion

In summary, the most reliable way is to buy or steal Grand Soul Gems, then convert them at one of the four necromancer altars during a Shade of the Revenant day by casting Soul Trap on the altar