Chin strap is a myth.
The gliders in the pics you were sent are not BB. They are classic greys.
BB's have very dark and thick markings, with very dark 'eye liner' around their eyes. Their fur is also a deep charcoal coloring and their bellies have an undercoat of dark grey/charcoal. They also get darker with age.
Here are a couple BB joeys from one of our BB breeding pairs:
Here is the coat/ striping of a couple black beauties:
Here are a few face pics (with flash so they appear lighter):
The "very dark" black beauty listed above is actually a "black face" black beauty. Very rare- there are only baby 3 ever and they are all father/sons. Generally BB's don't get much darker than the ones in the pictures I've posted.
Here is a BB next to a WF and a classic grey (just his nose) on top: