For a wetplate darkbox shroud I think the BK5 would work in a double layer either with itself or a fabric such as the aluminized nylon taffeta sold by Outdoor Wilderness Fabrics. I believe Andrew Moxom has used that latter combination in his darkbox. It's probably too reflective to use as an inner liner for a bellows and I've heard finding an adhesive that binds to it is problematic. The stuff is thin and would otherwise be fine for a bellows or perhaps as an outer layer combined with a tightly-woven nonreflective black fabric as the liner.
I've also seen some original Deardorff bellows fabric for sale on Ebay recently.