Just what IS that black film clinging to the inside of my skimmer?

[subedoo]

I don't know if you are using BBB or pucks but here is a testimonial . . .

I have used BBB since the pool was installed late last season. My pool has not seen a puck. I keep my chlorine on the "high side" of the recommended level (at 6 or 7) with my CYA at 35~40.