I use the fabric paint that's sold in craft stores for putting lettering on T-shirts by hand. I've also found it in Wally World. It comes in many colors, but I use black, of course. It's very flexible after it dries, and allows the camera to be folded up afterwards if not applied too thick.
I've used it to make the stiff and brittle bellows light tight on two 1910 vintage Seneca 8X10 views. It worked on a 1930s vintage Wirgin 120 folder, too.
Of course, this isn't really permanent. After a few years, I'll have to break down and do a complete bellows replacement. For the time being, it's working fine.