Here I think Mencken uses a pejorative not called for:

"Thus all hesitations disappeared, and there arose a national consciousness so soaring and so blatant that it began to dismiss all British usage and opinion as puerile and idiotic. "

Webster's 1913 Dictionary

Definition: \Bla"tant\, a. [Cf. {Bleat}.]
Bellowing, as a calf; bawling; brawling; clamoring;
disagreeably clamorous; sounding loudly and harshly. ``Harsh
and blatant tone.'' --R. H. Dana.

A monster, which the blatant beast men call. --Spenser.

Glory, that blatant word, which haunts some military
minds like the bray of the trumpet. --W. Irving.