Your question neatly encapsulates the two possible meanings of the phrase "Auguries of Innocence": first, prophecies made or omens seen by an innocent mind; second, prophecies made or omens seen about innocence itself. The two are not mutually exclusive, and both are integral to the poem. This dual aspect of the title is related to Blake's religious beliefs. Blake's religion was both radical and idiosyncratic: he abhorred institutional religion, particularly the peculiar theocracy of by the Church of England; yet he insisted that the Bible was foundational to his own life and worldview.
The life and death of Jesus perhaps illustrate how for Blake, innocence was not passive sweetness or ignorance, but a charged and powerful force. Being without sin, Jesus was of course innocent; even legally, Pontius the Pilate found him innocent of the crime with which he was charged. But Jesus was not an innocent. He was literally messianic, seeing the material and spiritual corruption of society with excruciating clarity, railing against that corruption, prophesying a reformed world, wielding tremendous power in attempting to bring that world about, and undergoing the cruelest sufferings to do so. That reformed world is of course meant to be innocent as well, in being both free of sin and full of power. This dual valence of innocence, as Blake uses the term, explains why this poem both augurs innocence (it foresees an innocent world) and is an augury from innocence (the world is foreseen through innocence).
Blake's vision endows innocence, usually seen as naïve, with immense potential. The paradoxes of the opening quatrain rely on precisely this vision: small and insiginificant things (a grain of sand, a mere hour) become powerful and boundless. The couplets that follow consistently exemplify this vision and reiterate this paradox. Just as the suffering of the innocent Christ is simultaneously horrifying and the emblem of a better world to come, the caged robin, the starved dog, the wounded skylark, etc. are simultaneously innocent sufferers and omens of an innocent world that is being built:
A Robin Red breast in a Cage
Puts all Heaven in a Rage
A Dove house filld with Doves & Pigeons
Shudders Hell thr' all its regions
A dog starvd at his Masters Gate
Predicts the ruin of the State
A Horse misusd upon the Road
Calls to Heaven for Human blood
Each outcry of the hunted Hare
A fibre from the Brain does tear.
Blake also makes use of innocent responses to suffering. It is childlike to think of a starving dog as equivalent to the fall of a country; experienced adults calibrate their emotional reactions differently. To Blake, who saw England and its associated Church as rotten to the core, the ruin of the state would perhaps be desirable rather than unwelcome—but the point here is that Blake insists upon childlike, immediate responses to suffering as the true auguries of both the innocent world to come, and the innocence needed to bring that world about.
Such immediacy admits of no doubt, as doubt interposes itself between vision and achievement. To lose this innocent vision is to lose the possibility of an innocent world:
He who Doubts from what he sees
Will neer Believe do what you Please
If the Sun & Moon should Doubt
Theyd immediately Go out.
Throughout, the couplets use paradox, childlike reactions, and proclamations of certainty to advance Blake's vision of innocence. The poem culminates in a restatement of this vision of innocence as a vision. Like innocence, vision is both what is seen, and what is seen with:
We are led to Believe a Lie
When we see not Thro the Eye
Which was Born in a Night to perish in a Night
When the Soul Slept in Beams of Light
God Appears & God is Light
To those poor Souls who dwell in Night
But does a Human Form Display
To those who Dwell in Realms of day.
These lines are extremely difficult to parse, but I believe they mean something like this:
Without this vision, all we have is darkness. That is, if you don't see this innocent world and what's needed to build it, you are both blind (lacking vision) and doomed to darkness (unable to realize this vision). Yes, we are all blind to some extent. For one thing we can't see what happens before birth or after death. But we all have divine vision that actually helps us see Creation as it really is, i.e., radically innocent. Because God appears as and gives sight to those without vision. And once we have this vision (in both senses of vision), will see that like Jesus, God is actually human.
Blake believed in the unity, not to say identity, of humankind and God. His heterodox Christianity posited a visionary mythology wherein radical innocence could both foresee and achieve this union. "Auguries of Innocence" is perhaps the most succinct expression of the vision Blake developed over his entire œuvre.