Commonly, a black line around the pool water line (and the inside of the skimmer) is a combo of people goo and lotion. At pools where many of the users are what my employees used to call "slip & dippers" (slop on the oil till they were slippery, then rinse it off in the pool, then repeat) the accumulation could be heavy.

I've also seen it occur when trichlor tabs with unusually high amounts of silicone mold release (from the process that presses trichlor powder into tabs) compound were used.

Regardless, you DEFINITELY want to clean it off if you can; time + sunlight + chlorine tend to make the black mark permanent.