I've done it both ways, and to be honest, I'm still not sure which is preferred. The factory used the black wtihout primer (as you can tell when you strip down a an original part (which still has paint residue). I like the reconditioning paint for it's gloss level, as I think that comes closest we can to what the factory did, so I use it in spite of it's lower durability (but how much weather, 'use' and 'misuse' are out cars going to see? not much I suspect...
PS. if you do prime, I'd suggest sanding most of it off, or applying it very thin if you're going to cover it with gm reconditioning black.