Blatant means done openly and without any shame. A blatant lie, blatant disregard, blatantly obvious.
The openness is what the word objects to: a blatant foul is one nobody even tried to hide. It intensifies the noun rather than describing it — blatant lie, blatant cheating, blatant favouritism. Note flagrant, which is close and means shocking rather than merely open; and that the word was invented by Edmund Spenser in 1596 for a thousand-tongued monster called the Blatant Beast.
1adjectiveblatantDone openly and without any shame.
“It was a blatant foul in about the ninth minute.”
blatant is a quality of openness.
Edmund Spenser coined the word in 1596 for the Blatant Beast in The Faerie Queene, probably from a Scots word meaning bleating. Few invented words have caught on this thoroughly.
Proto-Indo-European
*bʰleh₁-
Latin
blatiō — speak like a fool, prate
Scots
blaitand — bleating
English, today
blatant
coined by Edmund Spenser in 1596 in "blatant beast". Probably a variation of blatand (Scots blaitand (“bleating”)), present participle of blate, a variation of bleat, equivalent to blate + and. See bleat. In addition, it is suggested by Latin blatiō (“speak like a fool, prate”), which is rare, and so the similitude may be just coincidental. Compare typologically вопиющ, вопию́щий (akin to вопи́ть*).
Etymology from Wiktionary, CC BY-SA