As they get old they do degrade and heat will also cause "butt leakage".
Anything with a solvent , even hand cream can take some of the uncombined monomer out of the rubber especially on the surface and with this comes the carbon black that is used as a filler and to color it.
I bet if you put them in a dry place for a week to be sure they are dry then put them in a car on a sunny day , you would see some black rub off when they get warm.