Trying to solve this problem for the new house, but for now I'm stuck with it. The hoses on my handheld showerheads are those stainless steel wrapped ones. Over vinyl hoses I think. I constantly have to fight black mold forming in between all the little circular grooves. The closer to the bottom bend of the hose as it sits in its holder on the wall, the worse the mold.
Every week I get out my toothbrush (well, not the one I use for my teeth--please don't get another freakin' hygiene thread started here!). I get out the cleaning toothbrush and scrub the mold. Sometimes I use my microfiber cloth and fingernails and trace the little spirals. In between, when the hose is dry, I spritz it with hydrogen peroxide.