There are historical distinctions between blatant and flagrant, and English reference books still outline subtle differences between them, but in real-world usage the words are mostly synonymous. Both mean offensively conspicuous or conspicuously offensive. Offensively conspicuous is usually associated with blatant, while conspicuously offensive is associated with flagrant, but these two meanings tend to overlap. Combine this with the words’ similarity in sound, and the result is that most English speakers treat them as interchangeable.
Still, if you wish to go against the crowd and keep the words separate, think of blatant as a near synonym of brazen or obvious, and flagrant as a near synonym of outrageous or scandalous.
History
Blatant was invented by Edmund Spenser in his epic poem The Faerie Queen. It appeared in the phrase “blatant beast” and was reportedly (according to the Oxford English Dictionary) intended as an archaic form of bleating. The word soon came to mean loud or noisy, and from this the modern sense, offensively conspicuous, developed in the late 19th century.
Flagrant‘s roots are deeper. It goes back to Latin words having to do with fire, and its early English senses had to do burning, figurative and literal. It took its more modern sense—conspicuously offensive—in the 18th century, and this has been its primary meaning ever since.
Examples
Below are a few 21st-century examples of the words in action. Close scrutiny might cover subtle differences in how the words are used, but they are at least very nearly interchangeable.
James’ camp has denounced the suit as a blatant attempt to capitalize on the attention LeBron will receive in the run-up to his announcement tonight. [The Hollywood Gossip]
Princeton students, on the other hand, counted three times as many flagrant penalties by Dartmouth than flagrant penalties by Princeton. [Behavioral Economics and its Applications]
A city schools superintendent is independently trying to open up charter schools that would compete for the same struggling students served by her own district — a blatant conflict of interest, critics charge. [New York Post]
Anyway, how the FSA and its regulatory counterparts missed such flagrant abuse is one of the most important questions from this Libor scandal. [Telegraph]
With pressure for video replay mounting after two blatant missed calls at the World Cup, FIFA president Sepp Blatter said soccer’s governing body will reopen the issue after the tournament. [NBC]
The flagrant “F**ck U” caught in press shots is very clear and further supports opinions of Lilo’s roguish approach to her legal predicament. [The Wrap]
Blatant racism is easier to measure than more contemporary forms of racism because blatant racists will usually admit to holding negative attitudes and beliefs. [Applied Social Psychology]
A second United States senator complained Thursday about American military assistance to Somalia’s government, which the United Nations considers one of the most flagrant users of child soldiers in the world. [New York Times]