I've played with the Master Chief outfit on Xbox One but I didn't receive the Matte Black variant

To unlock the Matte Black variant for the Master Chief outfit, you must play on an Xbox Series X/S. Playing the game on an Xbox One will not grant access to the variant.

You can unlock the variant at any point in the future as long as you play a match using the outfit on one of the eligible devices.

You can learn more about it here: https://x.com/fortnitestatus/status/1871663664417198210 (Opens in a new tab)