I had this during my previous prgancy 2 years ago. I mostly had it under my lip- my whole chin area but my upper lip was a bit darker as well. No hair- just darker skin to where it looked like my face was dirty. It slowly went away after my baby was born.
When she was 15 months old, I got pregnant again and it immediately started coming back. It would get really dry and even kind of scally, so I just started making sure to wash/scrub well and moisturize afterwards. It is significantly better now. (A dermatologist told me they don't like the word "scrub". I use a sponge- not rough like "Buff Puff"- and an exfoliating wash (occassionally) as well as a regular wash.) I also make sure to always use sunblock.
I know it is not a HUGE deal, but just that slight discoloration made me feel ugly and dirty- especially because it showed up through make-up. (Mostly it seemed make-up did not stay on.) Good luck to you. I hope you find something that helps you. From skimming through your other replies, I can see that what works for one does not always seem to work for another. (I guess it's due to our individual reactions to hormones.)
Also, my vote would be to try the waxing or a cream hair remover to get rid of the hair. It is a risk but I find that even the few, hardly noticeable hairs I get around the corners of my upper lip make a diffence when they are gone. MY face just looks cleaner- more radiant. (Sorry if I sound like a commercial.)