It could be "pool goo," especially if your liner is new. I had a black film appear on the inside of our skimmer shortly after our pool was installed. I went to the pool store where we purchased the pool, and they told me it was "pool goo." I laughed and asked them if that was a technical term...and they unsmilingly said yes and even gave me a printout describing it, which I tucked in my pool folder! Here's what it says: "New vinyl liners often have a buildup of sticky substance that adheres to the bottoms of feet and sometimes turns black. This is plasticizer migrating from within the liner, and is usually only prevalent in colder water temperatures with new vinyl liners. Brushing and shocking can help reduce this buildup, as can increasing the water temperature. This buildup usually goes away on its own as the liner ages and the water temperature warms up."

If that's what it is, just wipe the side of the skimmer periodically, and eventually it will stop appearing.