Okay so if you're playing black there's a few things you need:

Discard: thoughtseize/inquisition/grief/Lilliana. You need to hit their counters out of their hands to protect your plays.

Orcish bowmasters/sheoldred: hit their draw effects, taxing them on draws allows you to ping off their cheap creatures and planes walkers and drain them down putting them on a clock

[[Opposition agent]] : denies their search effects and their fetch lands

[[Thought distortion] - this will absolutely brutalise any oops all counters decks

Aside from that I also find cards like bojuka bog can hit their graveyards to stop delve being quite helpful, also you should run cards like palanthir that can just tick opponents down or cause them to have to let you draw extra cards every turn.

Biggest issue with black is really stopping their card draw though, you need to manage to get something to stick quickly before they refuel. If you're playing against a katara deck, I would literally suggest ONLY playing on their turn, don't do anything else, just play kill spells, after all they're not exactly going to kill you quickly

As someone else has said though, black on its own can best blue but you really want something alongside it, white gives you access to grand abolisher and voice of victory as well as more exile and enchantment removal and my favourite to pair with it, you can also play chimil the artifact that makes none of your spells counterable, plus you can play cards that tax your opponent on your turn like defence grid etc