I checked my cars and found all the shocks to be too rusty to reveal any left over paint. I do have the original Pliacell shocks from my 66 Corvette with F-41 suspension and they are definitely gray with some blackout overspray. I also have some original shocks fom a 65 Corvair and they are gray. I know the 55-57 chevy spirals were gray and the NCRS says that 63 Corvette shocks were black.
It seems the most likely explanation might be that GM specs, as they often were written, may have given the supllier an option of providing the shocks in black or gray or may have not specified any particular color, just that they have an appropriate coating .