Is it normal for Teflon tape to discolor like this?
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.