Beardies can "
black beard" for a number of reasons.
1. If they are gravely ill, their beard will often be REALLY black. You would probably notice other signs if it were this, because being gravelly ill means "near death". A beardie can go downhill in a matter of hours, but you will usually know before the very seriously ill, that there is something wrong.
I am not saying that your beardie is dying, i'm just saying WHY they black beard heheh, it's probably a completely innocent reason!
2. Hormones. Their hormones can play havoc with them when they go from juvenile to adult, this can be them black bearing to boast to other dragons that they are bigger, better, stronger etc than them. A lot of throwing themselves around their enclosure, displaying, head bobbing etc goes on too. Often this happens with males when they come out of brumation.
3. He isn't happy with something that has changed either inside his house, or in the room that he is in. This CAN BE ANYTHING.
4. He's just not happy about something.