Today I was replacing a fitting on the house's water supply. I took off the PVC female adapter and the underneath looks to be Teflon tape that has discolored into black. I do not remember seeing this before. I wonder whether it looks normal to some of you or have you seen this before. If you can provide a chemicky explanation, that will be great!
I haven't seen Teflon do that before. It looks like a black pipe dope or sealer was applied over the Teflon to stop a leak.
There are many different colors of pipe sealers. According to @Kris, Rectorseal makes a black one, #7, for use with corrosive chemicals and active solvents and it also specifies for metal pipes only. We'll never really know what's there unless you get it analyzed. I'd clean off that part and replace the PVC female adapter and re assemble everything. The black material could even just be some black silicone caulk that was used as a last ditch effort to stop a leak.