While it is quite possible that there was a black 645 made by S&W, Id think it unlikely.
Member Model39 has one that was done up for him by a custom shop. I seem to recall a few custom shops, one being Novaks - which offered coating on mags to match a pistols - that did various treatments resulting in a blackened stainless gun.
Is the slide marked in any way besides the usual rollmarks? What is the serial prefix? If it falls into the regular 645 serial sequences, Id suspect a custom job. But Id buy it!
Hope this helps! Regards 18DAI