Your clutch maybe working for now , but that black goop you found is the rubber/Plastic moulding around your armature for the clutch . Very common problem on ag / industrial equipment.
Get the numbers and make of your compressor and have it replaced. Hardest thing on ac clutch is not enough air flow thru the condenser, a lot of idling in hot weather will drive the high side pressure thru the roof.
I like this analysis - could explain a bunch of those symptoms, and probably the deal.
