I would say, if you are looking for a black paint M3 to use and can get it for a decent price, maybe $1400-1600 (basically the price for a chrome one with a good paint job) - go for it. If you are looking for a collectible - stay away.
There are a lot of black M3 parts around, top-plates included. The brass strap-lugs are one thing that is difficult to "fake" though. You can always check with Leica Wetzlar about the provenance. Leica would happily turn your chrome camera into a black camera for a fee. Dealers and importers had orders and none in stock - so Leica filled the void. The bigger importers had their own engraving machines with correct numbers. They also had "blank" top plates, both chrome or black and you handed your camera over, they removed the top plate - had the number of the chrome one engraved on the black top - and supposedly destroyed the chrome one. It wasn't that expensive either - somewhere around $ 100 at the time.
Factory "non black paint list" bodies were often special order batches. Does not show up in the "official" list - only in the shipping documents. Remember that when these cameras were made - they were not collectibles, just working cameras.
As for black rings around the flash synch, I have seen both on black M2 and M3's (verified originals). I think it depended a bit on what was on the assemblers table at the moment.
