Go get some multi purpose spray paint or automotive paint from auto parts store. Most are resistant to heat far beyond what e cig
batteries create. Also, the 'nasty' stuff in spray or liquid paint is in the delivery system. Or. What keeps it wet. Once dry, most spray paints are no longer considered hazardous waste.
However, may I suggest stickers first. Large black film. It can be removed easily and re-applied when it gets worn out. The best I've found is 'black out tape' used on door frames by car manufacturers. Toyota has good ones. Just go to the local Toyota dealer and ask for one. Or let me know if anyone found a cheaper idea. Maybe ultra wide electrical tape?