Hi , I am not sure if i should worry, i never saw that before and i could not find any pictures that match what my bearde dragon look like when stressed. Only one circle on the left side of the throat get black. the other side stay light grey, as shown on pictures.
That's just incomplete shedding along the beard. I don't see anything to worry about. It is probably causing some grumpiness and beard flaring to loosen the skin. It should come free eventually.
Thank you, I realize now, just the beginning of a new shedding! the second one for Birdy. the reason for the blackening could be anything, he was outside basking in the yard, so it could have been a bird flying too low, or a loud noise (blower)