Due to unknown machinations of the universe, both grand and black soul gems can be successfully filled by casting Soul Trap on ketchup.

Joke and fake loading screens and screenshots from The Elder Scrolls IV: Oblivion, run by @ChrisOfTheDead.
General links and the actual Oblivion mod made from these jokes in my Carrd page:
Due to unknown machinations of the universe, both grand and black soul gems can be successfully filled by casting Soul Trap on ketchup.