Black Soul Gems

[Striker879]

If you already have a bunch of empty grand soul gems it's just a matter of timing. I find the most convenient altar is the one at

Fort Istirus

... when you're headed towards Anvil you'll see the

Shade of the Revenant

if it's the right day as you cross the bridge just past Fyrelight Cave. When I find they're 'on sale' I load up.

 

I believe you weren't on the mark about what souls the black soul gems can trap nosisab. Here's a quote from the Notes section of the UESP Wiki on Black Soul Gems:

Black Soul Gems can also hold non-NPC souls, so be careful when fighting normal creatures while you have empty black soul gems.

I always keep a supply of grand soul gems in reserve, along with the smaller sizes. Souls will always go to the smallest available gem that they'll fit into, and I like to have a variety of sizes of filled gems so that I don't waste much when recharging my weapons.