Is that blueish stuff fuel leaking out? I understood they couldn’t take off with a full load because so much leaked out when on the ground and cold, and the tanks only sealed up at speed with all the heat caused by friction from the air.
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?