As I was looking for bellows materials for my DIY Camera project (which is now on hold as I am traveling), a good option came from an unexpected place, a regular fabric store. The Ron-Loc blackout cloth, for $5.99 a yard is 99.9% lightproof and when painted black with fabric paint becomes 100%. I figured I'd share this with you, as many people seem to look for such a material. For bellows the material is on the thick side (at 0.015") but useable.
See the link in my signature for some other ideas.
See the link in my signature for some other ideas.