I think it would work well on a dull basecoat, but I wouldn't recommend it on plain plastic, because when one uses a sharpie on like, say, a plastic bag, the sharpie has this weird sheen, kinda like oil on water. Also, i'm not sure if sharpie is soluble in oil-based paint thinner (I assume it is not, but to me it would be very problematic if it were). Lastly and probably most significantly, many people (I included) prefer a shade of rubbery black with a little grey mixed in, because blackblack just looks too black (in my opinion), so sharpie might just be too black for some tastes.
But perhaps I'll have to give it a try!
FACEBOOK: Ryan Olson Thorgeirsson for pics of all my builds.
"There are two kinds of people in this world; those who put fries/chips on their sandwiches, and those who don't enjoy life."