Taken from my notes about breeding blacks...not sure who to credit, as it's quite old.
"Purple sheen is caused by too much black melanizer. Flat black is caused by too little black melanizers. The green sheen is caused by the correct amount. The black melanizers or enhancers are the genes that complete the colour black. It's basically a balancing act as far as breeding for it.
A dull or flat black bird crossed to a purple or plum hue bird will usually produce good green sheen. Green sheen is not terribly difficult to produce one you establish it in your line."
I've never had a plum hue black, but I have had dull blacks, and can confirm when bred to the right mate, the offspring all had nice, green sheen. This was with gold based birds, not sure if a silver base would change that or not.
Personally, if you have the space and feed, I would pair him with your best black hen with good, green sheen and your best dull black hen, and see what can be learned from the offspring. I know I've said this before, but I truly believe our birds are the best teachers. There are no breeding mistakes, only lessons and experience to be gained.
Good luck with your hatching!