Yeah, we didn't know what an "augury" was either, at first. We thought it was, like, an old-timey English girl's name—and Blake must have known a bunch of these Augury ladies, and they were all pretty innocent. But—get this, gang—that explanation is not even remotely correct. Yeah, we feel bad and kind of horrible for even mentioning it.
So, in pure point of fact, an "augury" is a sign or omen predicting some future event. Since these are auguries of "innocence," they're pointing toward… er, innocence. But Blake's not talking about the same kind of innocence as Britney Spears in "Oops! I Did It Again." After all, she said that she was "not that innocent." (Btw, we're dunking our heads in buckets of Clamato as punishment for bringing up Ms. Spears and Blake at the same time. But it needed to be done…)
Blake is, more likely, talking about the innocence that Adam and Eve experienced in the Garden of Eden, before they (according to the story) ate the apple and started feeling ashamed of their naked bodies. Blake is imagining a time—in the past, in the future, and even in the present—where human beings can achieve this innocence again, and attain unity with God. His couplets are meant to point people away from a life of violence and division, and hint at this better—more innocent—state of being.