The Eastwood metal blackening kit won't do a good job on previously plated (eg zinc) parts, even if you bead blast them first. A solution to this problem is to soak the part in diluted muriatic acid first to remove the old plating. Remember to neutralize the acid bath with baking soda (sodium bicarbonate) before disposing of it.
I've had very good results restoring my phosphated and blackened fasteners with a parkerizing solution from Palmetto Enterprises. A little goes a long way (i.e. a quart will last you a whole car's worth of phosphated parts) but you'll need a heat source and a stainless steel container to heat the solution.
Palmetto Enterprises
(864) 246-3836
Greenville SC
Website