My Reading of the Firearm act is that replica firearms are illegal to aquire or import, not own (I'd have to dig out the documentation if anyone wants it). So painting an existing gun black is not illegal as you aquired it legally and it's a modification.
However, Like above people mentioned... keep it out of sight and treat it like a real firearm. Buy the firearm act a replica weapon must be trasported like a real firearm (out of sight and locked in vehicle). And it is a mortal sin to do a crappy paint job.