Although this is a grammatically masculine adjective in French, it is given to girls as well as boys in Britain and the Netherlands. In America it is more exclusively masculine. It is not commonly used as a name in France itself.
In the United States it became more common for girls in the early 1980s, shortly after the debut of the television sitcom The Facts of Life (1979-1988), which featured a character named Blair Warner. The name left the American top 1000 rankings two decades later, but was resurrected by another television character, this time Blair Waldorf from the series Gossip Girl (2007-2012).
The Cornish form of this name, Jennifer, has become popular in the English-speaking world.
As an English name, it became popular due to the Fleetwood Mac song Rhiannon (1976), especially in the United Kingdom and Australia.