Appears to be toxic waste falling from a truck when Matt was pushing a man out of the way so he wouldn't be hit. That's based on the trailers.
Hasn't that always been the case? I'm honestly not well versed in the comics, so I don't know if there's a more nuanced version. I mean, "toxic waste" seems a bit vague for something that can imbue super-powers. Or am I perhaps reading it wrong? Was Murdock always a "gifted" person and being blinded simply brought his heighten senses to the forefront?
Come to think of it, in the comics is Murdock classified as a mutant or is he strictly of the "product of science" category like Banner, Parker, Rogers etc.?