donrae
Rest in Peace -2017
x2sounds like the gypsy face trait. It darkens the face only, no effect on body skin otherwise. Different from the whole body dark skin as on cemani or silkies.
Normally roosters lose the darkness and turn bright red when mature. Some hens may keep it, some don't.
Only sumatras are a breed with a standard calling for it. Other than that it's more or less random and usually on black, mostly black chickens of any breed/mix but it can show on a few other colors such as lavenders.