Why does internal combustion engine smoke appear blue? Could there be "blue molecules" in it? 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.
I can't really tell you the chemical difference but usually blue smoke indicates that oil is burned, too. Like some of the oil cycles get into contact with the engine and it is burned as well. And then probably heavier components of your gasoline once you start the cold engine. My Prof (lecture on Petrochemistry) once told us that there is indeed a difference between premium gasoline and the cheap one although the octane number could be similar. For the cheap ones they add a lot of stuff with a very low octane number and finally only a small amount compounds with higher octane numbers to get to 95. But it makes a difference in burning it. Maybe if the engine is still cold some of the "harder to combust" compounds don't burn that well and cause the blue color, too. 

It's probably due to Rayleigh scattering as you mentioned. The sky is blue and clouds are white, the normal exhaust gases should be water and CO2 so similar to clouds and the blue particles are smaller rests which appear blue.