Shades of grey-blue
The dictionary will tell you that French blue is an ultramarine pigment, originally prepared from powdered lapis lazuli. Modern interior decorators however have a very different view. The generally accepted description is a chic, almost sombre shade of grey-blue. It is sophisticated, rather than girly.
But what makes this shade of blue specifically French? According to Wikipedia its name is derived from the shade of blue associated with the heraldry of French kings since the 12th century. Referring to antique paintings, this is an ultramarine blue as in the French national flag of today. A brighter shade of this same blue is the ‘Bleu de France’ that was once the national racing colour of France. It is as clear as a summer sky.
An online search of images will produce many shades of blue that could also be variously described as wedgewood, periwinkle, petrol, aquamarine, ultramarine, pale blue and, to be tongue-in-cheek, even business shirt blue!
My very sweet vintage 40s hat of double bows, bought from the eBay store Tarnished Past Vintage Hats, is made from wool felt, with a bit of mink trim. The netting is trimmed along the edge with tiny light blue felt squares.