I don't have a RIMS system, but I do have a BoilCoil in my 9 gallon pot. It is roughly 30W/sq.inch (3750 total). I haven't ever seen anything black on it. I do get a hazy buildup that needs to be removed every few batches, but that is kind of whitish scaling type stuff that comes right off with vinegar. I also use a scrubby, blue or green or whatever I've got. What you have sure looks like scorching.