Constant small black on beard

Hi all,
I am new to the beardy world and recently got my first Dexter.
He has been with me now roughly 6-7 weeks, I am just growing slightly concerned as his beard is almost always slightly black.
There are sometimes where it clears completely, but majority of the time it always has a small tinge.

Some brief details:

Dexter is roughly 10" long from tail tip to nose.
He's fed on a primary diet of locusts, vegies. Have started to introduce other insects on a small scale (mealworms, crickets which I hate!) Calcium dusted 4x pw and vit 3x pw
Tank is 4.5x2x2
Uvb bulb exo 150
Basking spot kept between 40-43 Celsius
Hot area between 32-35 Celsius
Cool area 24-26 Celsius
Reptile carpet as substrate
Bathed 1-2 times pw

I am reading and reading from multiple different places and I just can't seem to figure out what it is. He had a good shed around 2 weeks ago and does look as if the colour is going dull on the tail/legs (same as when he shed before).

Any advice would be fantastic!!
Thanks

Here is a picture to clarify the above