I have seen vapor trails criss-crossing the sky and then the sky is completely hazey so there is something to it.
"It" being completely routine air traffic and given certain fairly common upper level conditions of temperature and humidity, their vapor trails will spread out and persist for many hours, which in a high air traffic area will essentially cover the entire sky.
Vapor trails are mostly water vapor, they are dispersed by upper level winds and turbulence. However that "blurring" is much less regular than the "donuts on a rope" trails.