I think you are talking about a scum line, oils and other sticky stuff get stuck to the plastic at the water level and turn black from dirt and debris. You'll probably end up with a black ring around the whole pool but I think it starts in the skimmer since all the oils and etc are getting sucked into that area. The scum bugs work well to prevent this, you can use a magic eraser on the plastic in the skimmer.
I'll second that motion! I too have the black ring in the skimmer (which I know is from all the suntan lotion getting into the pool), and even started to realize the tile area above the water line was getting hazy and covered over with "gunk". Based on both recommendations from this forum (THANKS!), I threw a scum ball into each skimmer and one into the pool itself (this one didn't do much help in the pool and wound up in the skimmer - that's fine), and am amazed at just how much that black line has now transferred to the scum balls; and then also used a Magic Eraser on the tiles and restored their shine instantly!! I simply swim around the pool with the Magic Eraser, wiping down the tiles and in about 10 minutes it's all gone!!
I understand there are various "enzyme" products on the market to help breakdown some of these oils and residue. I'd be interested to hear others opinions on any of these products. I haven't looked into them myself yet, so wasn't even sure if there is a particular one worth any while.
If you have lots of suntan lotion, tanning oil, bug spray getting into the pool the first thing to try is a scum bug/scum ball. These are foam things that you put in the skimmer that soak up oils. If there is too much oil for those to handle, the next thing to try is an enzyme product. They help chlorine break down the oils much more quickly so you never notice them. There are many many brands. ProTeam Power Enzyme is very reasonably priced, while Pool Perfect is usually easy to find at a store though more expensive.