U/R has been firmly established as the control deck of choice in Standard, and its reasonable matchup against Aetherworks Marvel decks has made it more popular in the weeks since the Pro Tour, but with such a narrow metagame to fight, perhaps we can do better.
Adding black gives you a lot of interesting new options, starting with Kalitas, Traitor of Ghet. With Zombies relying so heavily on rebuying their creatures against removal, Kalitas will dominate the matchup in any game where it goes unanswered. Liliana, Death's Majesty is there to recur it should your opponent have an answer, or maybe just a Torrential Gearhulk should you want to rebuy a removal or card draw spell.
These additional threats also let the deck apply more pressure than the U/R lists, which is important when the threat of Ulamog, the Ceaseless Hunger is there for any games that go particularly long. The added pressure also lets you take advantage of Ribbons, either closing games out of nowhere or forcing your opponents to play into your answers in an attempt to end the game before it comes to that point.
A third color is always going to make your mana more inconsistent, but in this case you gain an advantage to help mitigate the problem in the form of cycling lands. This list plays the full eight on-color cycling lands, so you have plenty of flood insurance and some added velocity to consistently find the right answers for your opponent's threats. It took a while for the Temple cycle of Theros block to catch on in the large numbers they eventually saw, and that may be the case with cycling lands as well.
But the primary driver for black has to be in the sideboard. Transgress the Mind and Dispossess are go-to cards to fight Aetherworks Marvel, and here they not only serve to answer the powerful artifact but also to gain critical information so you can optimally sequence your threats and answers.
We're at a point where the Standard format is firmly established, but that doesn't mean you can stop innovating. In fact, the clear target makes innovation easier, and this deck looks to be well-adapted to the current metagame.