This image of a Soyuz rocket plume was taken far downrange from the launch site and shows the second stage of the Soyuz, which burns liquid oxygen and kerosene, in the upper stratosphere or lower mesosphere. The visible plume here is sunlight reflected from soot generated in the engine. The size of these soot particles has never been measured and is not well known but the particles likely have a diameter of about 0.1 micron. This is the same size as smoke from a cigarette which also has a bluish cast, for the same reason. The tiny soot particles preferentially scatter blue light.
What does soot from rocket engines do in the stratosphere? It absorbs sunlight and heats the surrounding air a little bit. Is it significant? Climate models must be run to answer this question.