Noun; When a guy goes down on a girl that has left her tampon in for way too long and gets the tampon strong stuck in his teeth. He then proceeds to shake his head vigorously, in a panic, to free the tampon from his teeth, thus resulting in massive amounts of old black blood splattering on his face creating a "Black Beard".
In most cases a "black beard" can be avoided by recognizing the rotting odor coming from within.
Remember sniff first!
Rex was about to go down on sara when he remembered rule #1: Sniff First. Rex avoided a "Black Beard" by the hair on his chinny chin chin.