Think about it, Black Legion can be anything. Your warband could be actually loyal to Abandon, just using him for convenience, or only were conquered and changed colors to survive. They can range from Grizzled sons of Horus to newfags who just bailed from their old chapter. They can devote followers of the dark gods or absolutely hate the dark gods and reject that shit. Your warband could be savage raiders looking to plunder the galaxy or just some guys who just want to take the imperium down a peg. And while Black Legion favors close combat, you aren't committed to it. Your warband could focus entirely on tanks or something if you really wanted to. Due to how big the Black Legion is, your dudes could actually be anyone. The choice is yours all while rocking on of the best color pallets in the game.