There's SORT of a reason why Black Noir isn't in the battle itself. If you haven't seen all of season 3 of The Boys or spoilers for the comics, don't scroll down to the rest of this comment. See, in the comics, Black Noir is the one who kills Homelander. He rips his jaw off, similar to how Omni-man killed him in their Death Battle. Some may see it as a stretch, but Black Noir disappearing at the end COULD be foreshadowing him going to go kill Homelander, as he was the only one left alive at the end of the battle royale (minus Stormfront). Funny thing is though, this doesn't happen in the show. At the end of season 3, it's actually Homelander that kills Noir, by ripping his innards out. So, while not impossible, it's hard to believe it's going happen in the show. Not to mention the strength difference between the show and the comics, as Noir is nowhere near as strong as he appeared. So, yeah. Noir not showing up to fight is a touch upsetting, but he likely wouldn't be the winner anyway.