(More likely just dirt/tar from crawling around on the roof.)
That’s exactly what it is. Remember he climbed on top of the air conditioning units and then scaled up the HVAC piping against the wall. He got his legs/knees all grimy doing it
I do have one curious unanswered question.
The spectator video of Crooks first crawling on the roof appears to show a male individual wearing long white pants.
Makes sense - white roof, white pants for camouflage.
In the death photo, Crooks is obviously wearing short pants.