After a while of hunting and obsessing we finally found a way to replicate the finish of the original hardened nuts that fix the heads in place. Eastwoods Metal Blackening Solution diluted to the correct amount and cycled in just the right way creates an exact duplicate of a fastener that has received heat treating for strength.
The photos below show the before and after. The camera does not do justice to the finish of the nut. It is just the correct shade of black and should set off the engine bay quite nice.