You can trim the feathers with a small pair of scissors. As long as they are not newly growing feathers, they are fine to trim. New feathers, that are still in the shaft have a blood supply until they are fully grown and can bleed if cut then. Mature feathers won't bleed and it does not hurt at all. Many people trim vent feathers on birds that have chronically dirty butt feathers. The dark area may be bruising. How is he feeling/acting?