The possibility that it io related to chemtrails is likely - over the years (15) , chemtrails have been examined and crazy amounts of metals quite toxic to be exposed too, have been discovered , as well as molds, fungi, dessicated red blood cells, many other kinds of pathogens .
As well they have been found to contain special polymers (Plastics) that sheath various ingrediants for weather proofing , sun , mositure , tempatures variables etc.
That being said , I agree that the blackness (typical carbon build) up would be created by the soot from a lighter or butane torch as well as picking up pollutants as it falls through the heavens, which could concievable blaclken at behest of flame, , but a clear blue natural gas flame without yellow or orange flames , should not blacken clean ice, (no soot),however it probably shouldn't smell like burning plastic . I understand the idea of sublimation ,melting into h2o vapor with no observable water droplets . A possible good way to see the sublimation vapors happening is to hold a piece of glass (clear plate) at an elevation above the flame and ice where it wouldn't heat up rapidly, and look for condensation drops on the plate