While restoring a 10x8 Agfa Ansco two years ago I had to re-glue the bellows, I noticd that the inner cloth had faded quite badly and was nowhere near to being black eough. I used a semi-matt black acrylic paint to rectify this, it only needed a very thin application and is fully flexible. Choice of acrylic paint is quite important as some will cause the folds to stick together.