I left some of my qds soaking in Star San overnight and found the metal on the thread colored black. Why did this happen? I think I may be able to buff out the black stain but it is very weird.
i’ve done that too. no big deal. can’t tell you exactly why, but likely because it is an acid based solution. it just reminded me that Star San only needs a 3 minute contact time to sanitize. soaking metal overnight turns stuff black. scrub it. some of it will come off.
I once discovered that idophor turns silver purple. :o it was my wedding ring and my wife was much happier when it eventually wore off.
Why are you soaking over night in starsan?
When I forget I’m doing it. 
+1
I totally forgot that I had left them in the StarSan bucket.
It happens… Happens even more often the older you get. Just ask me what vinyl tubing looks like after a month in starsan… :o No, never mind. I’ll tell ya. It’s like a snake that just crawled out of a swamp. Slimy & gooey… The only good thing was it was really sanitized… :
I got a vinyl transfer hose that’s been soaking in starsan for months–what was clear is now opaque and whiter than white.
Yes, I still use it for transfers w/o problems.
My ignorant guess is that the metal in the threads was a bit worn, so the underlying metal was exposed. Star San is acidic (~50% phosphoric acid), so it corroded the metal. The black stuff is some variety of metal oxide.
Unless the oxide looks like it’s coming off, or is in direct contact with your beer or wort, I wouldn’t mess with it.
I’ve got some disconnects that are really old and have had the plating on the threads completely eaten away by Star San I’m guessing. I’m sure I’ve forgotten them in the bucket a few times for hours or overnight, but because those threads don’t contact the beer it doesn’t really matter. I should take some pics. I’m just impressed they have lasted this long without me breaking them or something