This is how I repaired a Mamiya 6 bellows.

On the inside I glued an oversize patch cut from black gaffer tape. On the outside I applied many coats of a product called Plasti-Dip (black, of course) which I first thinned with naptha. I let each coat dry for about an hour before applying the next until it all looked fairly well blended in. Plasti-Dip is basically a liquid rubber.

I used a small paint brush to apply it and also painted it on the frayed corners of the bellows which seemed to work well at sealing and strengthening them. There were no light leaks afterward and the bellows folded normally.