As someone that ran a black discard package, here is my long answer:
The best 1 CMC black discard that isn't commonly played outside of the 3 is Blackmail - its a lot better than you think. For 1 CMC, I primarily stuck with Divest in powered environments as creature ETB have been a problem + artifact removal has been harder to come by. It was a pretty good card in alot of fair decks.
2 CMC black discard (unconditional) are generally good filler cards - they're literally playable in every single archetype - Combo, control etc. (even good sideboard pickups for aggressive decks) and they're really good for supporting slower cards like Experimental Frenzy, Season's Past, Bedlam Reveler, Kroxa, Titan of Death's Hunger if your cube wants to go in that direction.
My cube needed to support a mix between grid drafting, 4 player drafting and 8 player draft + my environment was combo heavy, which is why I opted to add a heavier 2 mana discard package. They were just good answers overall in a format that often lacked good answers and were good playables during drafts were a much smaller draft pool was opened.
I personally wouldn't play conditional black discard unless your archetype needs the graveyard for Delve, Escape etc. or if you have slower cards to recoup the card disadvantage.
But in general,
If I had to pick, I would probably choose the 2 mana discard spells - but this depends more on your cube's goals.