Pure blue sky in Spring
This is related to a similar question on April 29th in that the apparent colour of the sky is mainly caused by the absorption of different wavelengths of light by a wide range of atmospheric pollutants. I strongly suspect that if you compare the antecedent rainfall with the colour of the sky (in a given direction at a given time of day) you will find a marked correlation. The rain scrubs the atmosphere clean, so, facing away from the sun, you are likely to see a brighter blue after any period of significant rain.
So what is the typical amount of rain in spring, summer, autumn and winter, in your part of France?