I'm pretty sure that 'unblockable' is meant to mean it was the only magic known to have no counter-effect, or counter-spell. Now Harry has discovered the true Patronus charm is the counter spell to Avada Kedavra. It makes sense when you think about it, which I'm sure is why Eliezer included it in the first place. The Dementors are voids of nothingness, into such nothingness tumble all living things once their life is extinguished (according to present evidence, anyway); in other words, the Dementors are parts of Death, but are not Death Incarnate (which can be summoned according to a Dark Ritual Quirrell read tell of as a spritely young lad). Thus, if the Patronus charm has the ability to repel a piece of Death, then in accordance with magic's apparent system of dualities, the Patronus charm must represent the opposite of Death: Life.

All but Harry cast their Patronus using memories, figments of the mind based upon reality; because they only conjure a thought reminiscent of all life can be, they can only manifest a fragment of life force to shield themselves from Death - an imperfect shield, permeable to Death.
Harry recognizes the two poles of reality: Death, or absence - and Life, or presence. Harry brings to mind all that reality really is to us, namely all that a life can ever possibly experience, and pushes that in the face of the part of Death that is Dementors.* Harry has this ability because he strives to and greatly succeeds in deceiving himself of nothing (he's not perfect - yet); through rationality he is able to have an accurate enough map of the territory that his conjured thought actually is a picture of all life can be, and so he can manifest the entirety of his Homo sapiens sapiens being. His Patronus not only represents but is pure life force, so it can overwhelm and obliterate imperfect representations of Death id est Dementors.

Likewise, as both Dementors and animal Patronuses are imperfect representations of their respective pole, their effects more or less cancel; an animal Patronuses' ability to repel a Dementor is in apparent proportion to its representative accuracy. Harry's true Patronus is the the form of Life itself, and so he can destroy Dementors and cancel Death; Dumbledore accepts most of the Life dimension of reality, and nearly all of the Death dimension of reality, and so his representation of Life, his Patronus, is the strongest and brightest yet mentioned.

Now comes the tricky part of this hypothesis. The Killing Curse is equal to death in that if struck by it, assuming no inherent rule of magic regarding altruistic protective love interferes, you will die. The true Patronus has demonstrated the ability to overpower and destroy imperfect representations of Death, as it is the duality opposing Death in true form. However, the Killing Curse and the true Patronus cancel, so it must then follow that the Killing Curse is Death in true form, but not Death Incarnate.

However, this theory is completely blown away if the true Patronus doesn't cancel the Killing Curse at all, that Harry blocked Quirrell's Avada Kedavra merely by effecting an interaction of their magic. Whether the true Patronus can truly block the Killing Curse remains to be seen.

*From what I've gleaned from the text, I do not think Harry's statement, "I think of the absolute rejection of Death as the natural order [paraphrased]," accurately reflects his thought processes preceding the casting of the true Patronus; to reject Death is to acknowledge it, and I don't think Harry acknowledges Death at all when casting the charm. Furthermore, I do not even think it possible to cast the true Patronus when thinking of Death at all. When casting the charm, Harry calls to mind all that Death isn't, and all that life is; he thinks purely of life, with not even a single shard of a thought of Death in mind.

As an aside, I find it particularly clever how in contrasting everyone else in the wizarding world, or agents of general humanity, with Harry, an agent of rationality, Eliezer characterizes animal Patronuses as akin to the self-deceptions and biases of most of humanity, and the true Patronus as akin to the power earned through the diligent application of rationalist techniques to everyday life, through elucidating the difference between animals'\ capacity for understanding and human capacity for understanding.
\Excludes humans as a subset; the sentence becomes awkward otherwise.