Test and see if it's a problem. Dying your bellows some darker shade of black would probably reduce internal flare (on the other hand dying them purple might increase their "flair"), but there are other approaches.
The easiest way to reduce internal flare is to restrict the image circle with a compendium lens shade. This will usually improve contrast with any format--even 35mm. The shade should be set just short of where it vignettes.
Using a larger format camera with a reducing back also reduces internal flare, but this gets to be an unattractive proposition at 11x14".