How old is your beardie? How long have you had her?
A lot of times, when they're young (less often when they're older, but not "uncommon") if their body temp is low, the stress marks and the
black beard come out. (likely because darker colors absorb more heat.)
My suggestion would be to check the temps in your viv. Basking side should be around 100-110, cool side 80... I'd also check to make sure that your beardie can get close enough to the heat
If this is the issue, my guess for the marks going away after a
bath would be that your beardie was able to absorb enough heat from the
bath water to not need the boost
there are a number of issues that it could be, but that one is the simplest.
it could also be the bulbs you're using in your viv.
What lights do you use? (Basking, uva, uvb: brands please!)
if they're the coil bulbs, it could be that the bulbs you're using are irritating her eyes. (there are TONS of ussies with compact and coil bulbs.)
those are the two most common issues that I've seen
I hope this helps!
Let us know if you have any more questions