quadratec has a thing called "forever black" that i got and its amazing - basically it's black shoe polish but for plastics and vinyls.

make sure you clean your flares really well and let them dry before you put it on. a good technique that i use when i put forever black on my jeep (i did it two summers ago and JUST had to do another coat) is you dab a lot of it on the end of the applicator pad so its foamy and then spread it like you're painting the flares -- forever black is good stuff

once you've got a coat of it on there, let it dry for a while and then go back over the spots that are uneven - basically if you build up the coats it will look more black and black each time you do it - you'll be pleased with the results