The work jacket is a bit like the uniform of the worker, the manual worker. Factory work clothes became widespread at the end of the 19th century. It is at the beginning of a blouse with a belt which protects from dirt, then of a jacket with trousers often having pockets to carry tools. Clothing should be comfortable, sturdy and easy to wash.
While on the other side of the Atlantic, jeans made their appearance in the form of jumpsuits and became the uniform of the American worker. The unions in France are campaigning for the bosses to equip their employees with an outfit, then much later for the company to take care of the maintenance of the blues.
Leaving the Renault factory (archive)

Overalls in France

The post-war United States financed, among other things, all the Massey Ferguson tractors to revolutionize small-scale agriculture and make it industrial. The peasants took loans subsidized by the state to acquire this machine allowing them to multiply the harvests.
The latter became more or less able to finance the studies of their children, these same children left far from their homes to acquire knowledge. This change of life has created a generational shock, the children prefer to live in town, the blue side of dad's work ending up being neglected.
blue is work
While the unions are campaigning for the right to work overalls to be respected, some workers refuse to wear them and prefer to work with their own clothes to contest belonging to a social category and to the factory.
The symbol of the blue color of the work jacket in post-war French society is synonymous with laborer, belonging to a working class and not intellectual. Hard work is a symbol of popular social class.

The choice of material:

Moleskin (or moleskin ) is a tightly woven cotton canvas , covered with a flexible coating and a flexible varnish imitating the grain of leather .
Moleskin is often presented as the French counterpart to American denim , both frequently dyed indigo and intended for the same purpose.
French moleskin clothing can however exist in other colors depending on the public for which they were made: white for painters, black for carpenters, blue for factory workers.
Drill work jacket
The work jacket is made of very solid drill cotton. It is an economical alternative to that cut in moleskin fabric. Always in Prussian blue