Like mentioned above, heat will re-blacken the plastic but you have to be quick. I've seen guys use automotive headlight repair kits to fix plastic as well but most are just thin clear coat. A quick wipe with lacquer thinner may work too but don't let it soak in, just a damp rag and a quick wipe. The dealer I bought a car from back in the late 80's was doing that to rejuvinate faced black and gray trim pieces made of plastic.