R/B decks are universally considered the most powerful decks in Standard. Combining the early and late game threats of red with a light black splash allows you to still cast your RRR cards. The mana is good enough and the cards are powerful enough that you can not only play a reasonable number of black cards in your R/B deck, but many with BB in their casting cost as well.
This is still a Goblin Chainwhirler deck even if you won’t be casting it on turn 3 consistently. Chainwhirler dominates Standard, leaving you with a 3/3 first strike after clearing off some of the smaller creatures. You don’t have Soul-Scar Mage here to combo with it, instead you opt for more individually powerful cards.
The black creatures provide this deck with the ability to get ahead on cards. Glint-Sleeve Siphoner is certainly made dramatically weaker by the number of Chainwhirlers in the format, but it can totally take over in a number of tougher matchups versus control decks. Ravenous Chupacabra provides a way to deal with some of the bigger creatures in the format, and even leaves behind a 2/2 for your troubles.
Pia Nalaar and Glorybringer round out the creature suite here as every single creature provides some additional value for your investment. Pia gives you multiple bodies and some evasion, and Glorybringer demands immediate answers before it destroys the opponent’s board and life total. While this deck loses out on some of the resiliency of Scrapheap Scrounger and the combo potential of Soul-Scar Mage, all of the creatures are individually excellent and require no additional support.
With all of these creatures with enters-the-battlefield abilities (or haste), Liliana, Death’s Majesty becomes incredibly strong. A planeswalker that can get these creatures back from the graveyard lets you cascade card advantage. Lili enters, you get a creature back, you get the creature’s enter the battlefield ability, and you get a planeswalker and creature left behind for them to deal with. Good luck!
Chandra, Torch of Defiance is still your every day card advantage machine, mana generating engine, and removal spell. Chandra takes over the game on empty boards and can completely tip the scales in a matchup where the opponent needs to focus on dealing with your card advantage creatures.
The removal suite for red decks was already good. Magma Spray can trade up on mana while dealing with problematic recursive threats and Abrade doubles as a removal spell and a way to deal with Vehicles and God-Pharaoh’s Gift. Adding black means that you also get to play a full set of Vraska’s Contempts to deal with big creatures, planeswalkers, and the many indestructible threats in Standard. This clean answer to The Scarab God can be a real game-changer versus an otherwise tough-to-deal-with threat.
The extra black means that you get to utilize Cut // Ribbons as an early removal spell that will end the game later on. A single copy of Arguel’s Blood Fast gives you another mana sink, additional mana, and a card advantage engine in control matchups.
The mana base is definitely weaker when trying to play double-black spells with your triple-red Chainwhirlers, but the mana in Standard isn’t bad right now. Siphoners combined with Hubs can give you extra cards, but it also leaves behind solid fixing should they deal with the 2/1 creature. With three other R/B dual lands in the format at your disposal, R/B Midrange has a lot of power at its disposal.