35mm B&W negative film IN GENERAL has a Grey (sometimes Blue) tinted base for anti-halo purposes. 120 and other roll film sometimes has a tint, but sometimes does not. (folks have complained about the Blue tint on Some Foma Made 120 film here in the past) If you are used to 120, SOME 35mm will have a darker base.

Light pipe effects are Quite strong with Polyester base film. I play with movies and if you hold up a reel of Polyester 16mm Movie film to a light you can see the light strongly, while a roll of Acetate film will look dark.. Film which gets fogged from Light Piping will look dark around the perforations. I have seldom seen fog on acetate film that can be attributed to light pipe effects., although I have seen fog from worn or damaged cassettes. I use a twin check tap on my reusable casettes and junk any that correspond to rolls of film with light trap related fog, to try to keep them for getting bad enough to ruin shots.

Dye that washes out in processing is not part of the film base, but may be part of an anti-halo treatment or part of the dyes used to make it panchromatic. The strongest Dye is on the EFKE films which have a purple dye that is visible on the back before processing. The most famous Anti-Halo is the REM-JET used on Motion Picture Negative Stocks ( and Kodachrome) which is a black coating that is removed by a special step in processing.