Haraka - some pretty credible statements at the time (I'll see if I can find the thread) from a few people who obviously knew a lot more than they were saying that there indeed is a Stealthy variant of the Blackhawk. One even mentioned the Project name.
It's been mentioned that 160th SOAR do a lot of mods in house.
If there are only a handful of Stealth Blackhawks - even just one or two - then it's entirely possible it's a type that been concealed until now.