Full disclosure: I mostly do this because I'm priced into all my archetypes being 3 color, and I do lean on custom cards here a decent amount, but I run BR Prowess as part of Grixis Prowess
If you're going to be jamming your deck full of non-threats (ie instants/sorceries) theres a few ways you can go about it. You can jam a ton of air (cantrips) and lean hard on the few threats in your deck to affect the board enough to get you there, you can jam instants/sorceries that are threats like
lingering souls, or you can
thoughtseize your opponent
Red Prowess cards are mostly obvious at this point, that's well trodden ground, so I'll focus on black cards I like for this deck:
Big Draws:
I double up on Bob, and it's the ultimate card in an aggressive deck like this that wants to trade cardboard for cardboard while beating down.
Sedgemoor Witch is really good, the lifegain on the tokens is non-negligible, she's got a real body you can attack with, and the ward is exactly what you'd want it to be.
Knight is a generically strong card, but we get the added benefit here where if you're running shocklands, you can grow this guy without attacking by paying 2 life to both your land and a
thoughtseize. You are gonna want some cheap threats you can ride to victory almost alone (see delver of secrets) and this guy fits the bill quite well
I run some custom criminals as well, but this guy has been really solid. It may look like a
gravecrawler, but since you can trigger it with near enough anything, it's less about putting loops together and loading the same creature into
goblin bombardment 12 times and more about a consistent threat that can net you an extra card every now and again as you keep throwing removal spells around to keep your opponent on the back foot.
I'm currently on these 4 as a spread @450 cards, so I'm likely due for a 5th and 6th at some point, as most black decks want them.
I really do think the universality of targets these cards provide is crucial,
duress and
ostracize really do whiff a lot or present some hands where you get a card, but it's a non-decision where you didn't really gain anything, might as well have played
Raven's Crime (A card I did cut for similar reasons, retrace be damned)
You are going to want cheap removal. Dial that down to what your cube wants, but if all your 1-for-1s cost 3 mana this deck aint gonna work. Other hits include
Vendetta,
Snuff Out,
Dismember (though more technically, this is way more of a green card than it plays here).
As a last note:
Hymn to Tourach would be good here, but I don't run it at the moment. Every other 2 mana mind rot I've ever tried has been really bad, so sadly I do think the feel-bads do provide function here. Take it or leave it.