This one is a matter of personal preference, but there's a colossal crop of players who prefer playing FPS games with mouse and keyboard, primarily for increased control and accuracy. With the addition of a battle royale mode, mouse support for the Xbox One version of Black Ops 4 has become even more of a necessity.
It's obvious why Fortnite was one of the first games to receive mouse and keyboard support. If you've played both the PC and console version of the battle royale king, you know it's easier to throw up walls or use items quickly by using side mouse buttons or specific key bindings that you've set yourself instead of relying on the game's vanilla bindings.
Blackout may not have the same building aspects as Fortnite, but there are still plenty of other battle royale elements that would make this a much smoother experience with a keyboard and high-end gaming mouse.
That's not even mentioning the boon this would be for traditional multiplayer matches.