I don’t see any “blue-ish” material. I see, just rear of the turbines (jets) is appropriate “heat reflection” exhaust; an indication of full-fuel combustion (and at that height, extremely fuel efficient!).
Where might you have found the material you describe?
Ahhh...I see now. Material on the flat surfaces.
I’d be more inclined to say that is transient ice-up other than fuel. As you pointed out, it may leak on the ground and until the bird is on full flight.
This photo would be assumed to be in full flight; that’s why I say “transient ice”.