Yes, they do exist. I've seen one.. It looked kinda like a matte-finish 586. According to the SCSW3, there was a run of 2,876 in 1989. They came with 6" barrels, .375" semi-target hammers and .312" smooth combat triggers. Product code is 104248.
As far as value.. hard to say. There aren't a lot of them out there. But, the one I saw for sale was tagged at less than a conventional 686. And it had been sitting there for a while..
I believe there are also some 4" versions of this gun out there, done after the first run of 6" barreled guns were made. I may have this wrong but I thought some of these in 4" were bought for issue by some police agencies. As said they are not common, but not pricey either. I just saw one change hands a week or 2 ago, I think on the other forum.
hesco: Weighing in late on this, but seldom see anything about a Midnight 686. I
bought two, one with a 6 inch barrel and one with a 4 inch barrel. I guess it was
the novelty of having a black stainless revolver that had me forking over the bucks. I traded or sold the 6 inch, long ago, still have the 4 inch. I put the 4 inch
in a leather holster and had it stored for at least a couple of months. When I
pulled it out of the holster the acid in the leather had taken off most of the Midnight finish from the wide sides of the cylinder. My guess is the Midnight finish
either didn't hold up too well, or the demand wasn't there for a black stainless. On the up side both were very accurate. . .Hope the info is helpful.