Removing sand-blasting residue from window glass
You wouldn't be asking if it just wiped off, which it would if it was just a little bit of low-energy dust caught by the wind. 

This tells me the glass was hit by high-energy sand particles, i.e. caught overspray from the blasting proper.  Sandblasting damages glass, and your contractor should've known that.  He has etched the glass (by roughening its surface, removing the gloss, which sandblasting does quite sincerely). I don't believe it can be recovered, and will need to be replaced.