Darkening is caused by, basically, rust. Tiny capillaries get ruptured during PE work. Over time, the iron in the small amounts of blood stain the surrounding tissue.
Some guys have members darker than the rest of their body to start with. Others don’t care about the darkening. Changing your PE program is certainly an option. Some guys wait until they meet their goals, then do one of the chemical peel routines to replace the darker skin with new, natural-colored skin.