1
general::
Say abruptly or inadvertently, utter without thinking. For example, Unfortunately he blurted out how much he hated formal dinners just as his hostess walked in. [Late 1500s]
American Heritage Idioms
2
general::
Phrase(s): blurt something out (at someone)
to say something to someone without thinking. (Usually to say something that should not be said.) • It was a secret. Why did you blurt it out? • Why did you blurt out our hiding place?
McGrawhill's American Idioms And Phrasal Verbs