I think Thassa is able to get there as a blink support piece due to the 3 additional upsides it carries, chiefly the indestructibility. Teleportation Circle breaks records in efficiency for white recurrable blink, but it still “does nothing”.
Gideon’s just plain good. Profoundly boring, but considerably effective.
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.
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.
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I'm actively maintaining a comprehensive article to help explain to new cube players how some complex vintage level cards work in a cube environment.