France has been unified as a country for longer than many European ones, and the 19th century didn't see the development of a specific French national costume (unless you see the international influence of French fashion as a costume issue).
Even though there are some surviving bits of regional costume, there is no one French national costume. These regional costumes were largely codified in the 19th century, pretty much as they were disappearing from general use. Some regions have held on longer, for instance it's less unusual to see older Breton women in lace caps on holidays than to see Alsacian women in their corresponding butterfly hats.
The things that American think of as 'national' were just fashion fads (often based in former regional costumes) at times when Americans were around France a lot. So the post-WWI fad of Basque berets is perceived as specifically French, even though it's equally Spanish, and nobody bats an eye at pairing it with the Picasso-led fad for Breton striped fishing shirts (or the overlapping 50s bikinis). To this day the French are much more fashion-oriented and conformist in clothing than Americans, which can lead to endless books about how little black dresses or tan raincoats are national costumes. The authors of these books usually go home and ignore the fact that the following year the same people will all be wearing red dresses and black quilted coats, or whatever.