Agree, Grasp of Darkness is strongest version we will see in a while. 1B instant speed version is not desirable imo. At sorcery speed that card is the same level as Lava Coil. One can argue black should have better spot removal then red but the instant speed upgrade is probably too much.
There is also the argument that Red should have the best CMC 1 spot removal, black the best CMC 3 and both colors "meet" at CMC 2. This way both UR and UB control decks are promoted and UBR gets the best spot removal out of all control decks, balancing it against UBW which already gets unconditional sweepers + both enchantment and artifact removal.
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Standard - Serious U Tempo W Weenies UR Izzet Drakes Standard - For Fun / Testing BRG Aristocrats GUR Gates WU Tempo Skies WB Knights GWR Nikya Ramps Huge Dinos
Mainly, there is the question of how aggressive and how large the set's threats are. A set with multiple aggressively costed N/4 creatures will want a more aggressively costed -4/-4 removal spell, for example.
Rarity also matters. A higher number of the aformentioned threats at common will encourage the removal to be common as well.
Also depends on the other removal in the set. Red will usually take care of the smaller creatures. I think that in a set with Lightning Strike, for example, will be more comfortable with black removal that gives -4/-4 or -5/-5. Similarly, if there's other black removal already in the set at a certain mana cost, that pushes -N/-N removal away from that spot. For example, you're less likely to see Murder alongside Ob Nixilis's Cruelty, for example.
As for speed, I think it depends on how significant the hasty or flashy threats in the set are. Riot, for example, is the kind of mechanic that would push one toward using more instant-speed removal.
As for my personal preference, I like sets that have more aggressive threats, so I would not want to go above 3 mana for a -4/-4 spell. I would also be partial to instant-speed, but it really depends on the context. I will say that Ob Nixilis's Cruelty is surprisingly efficient. I wouldn't have expected a -5/-5 effect at instant with upside to be only 3 mana and only one black symbol.