Four-stroke and two-stroke gasoline engines as well as diesel engines can all emit dense clouds of blue-tinged smoke under various transient loads or incorrectly tuned conditions.
Raleigh scattering - in this case probably from particulates of a limited range in size - strongly favors short wavelengths, so there is a possible physical explanation.
But I would like to know if there are any chemistry effects that could lead to or contribute to the slightly blue color sometimes seen in engine smoke under these conditions.
Could there be large "blue molecules" in the smoke? I don't think so; usually gases are transparent, though there are a few notable exceptions like molecular iodine and chlorine, but I'm not sure.
above: Iodine gas, from here
above: Chlorine gas, from here
below: snapshot sitting on a bicycle, looking at adjacent scooter while holding breath and waiting for the light to change.
below: screenshot from the YouTube video Big Engines Starting Up.