It can be blocked, but not reliably
When Avada Kedavra is referred to as "unblockable," this refers only to magical protection, and only to the knowledge of the people referring to it as such. Bartemius Crouch Jr. is our main source on the unblockability of the Killing Curse. We can presume that, as a noted Death Eater, he knows what he is talking about.
The only cases, beside sacrifical magic, where the Killing Curse is shown as being blocked were rather rare situations:
The Elder Wand would not kill Harry, and so when Voldemort attempted to kill Harry with it, his spell rebounded off of Harry's and killed him.
The bang was like a cannon blast, and the golden flames that erupted
between them, at the dead center of the circle they had been treading,
marked the point where the spells collided. Harry saw Voldemort’s
green jet meet his own spell, saw the Elder Wand fly high, dark
against the sunrise, spinning across the enchanted ceiling like the
head of Nagini, spinning through the air toward the master it would
not kill, who had come to take full possession of it at last.
—Harry Potter and the Deathly Hallows
Because Voldemort and Harry's wands shared cores, when Voldemort attempted to kill Harry, shortly after the former's resurrection, they experienced Priori Incantatem.
“They will not work properly against each other,” said Dumbledore.
“If, however, the owners of the wands force the wands to do battle . .
. a very rare effect will take place. One of the wands will force the
other to regurgitate spells it has performed — in reverse. The most
recent first . . . and then those which preceded it. . . .”
—Harry Potter and the Goblet of Fire
In this case, the spells from the wands also collided, and due to the shared cores some rather odd effects occurred:
A jet of green light issued from Voldemort’s wand just as a jet of red
light blasted from Harry’s — they met in midair — and suddenly Harry’s
wand was vibrating as though an electric charge were surging through
it; his hand seized up around it; he couldn’t have released it if he’d
wanted to — and a narrow beam of light connected the two wands,
neither red nor green, but bright, deep gold. Harry, following the
beam with his astonished gaze, saw that Voldemort’s long white fingers
too were gripping a wand that was shaking and vibrating.
—Harry Potter and the Goblet of Fire
Interestingly, the common factor here seems to be that Avada Kedavra collided with another spell. Dumbledore's phrasing is telling: the wands are "doing battle." In second case, Harry and Voldemort matched wills to see which wand was victorious. In the first, Voldemort's wand essentially gave up the battle, since its allegiance was to Harry.
It seems likely, then, that Avada Kedavra can be blocked if it directly collides with another spell. However, we are not shown what the results would be, except in two unique situations.
One might suppose that the Shield Charm can protect against Avada Kedavra, based on this scene from Deathly Hallows:
He was searching for Voldemort and saw him across the room, firing
spells from his wand as he backed into the Great hall, still screaming
instructions to his followers as he sent curses flying left and right;
Harry cast more Shield Charms, and Voldemort’s would-be victims,
Seamus Finnigan and Hannah Abbott, darted past him into the Great
Hall, where they joined the fight already flourishing inside it.
—Harry Potter and the Deathly Hallows
This seems unlikely, though. Shield Charms are one of the most basic self-defense tools, and certainly Avada Kedavra would not be referred to as unblockable if such a generic counterspell could block it.
“So we’ve expanded into a range of Shield Cloaks, Shield Gloves . . .”
“. . . I mean, they wouldn’t help much against the Unforgivable
Curses, but for minor to moderate hexes or jinxes . . .”
—Harry Potter and the Half-Blood Prince
Perhaps Voldemort was casting spells other than Avada Kedavra (unlikely, but possible).
More likely, it is the same explanation as before: Harry's sacrifice, which he mistakenly attributed to the Shield Charms.
"—I meant to, and that’s what it did. I’ve done what my mother did.
They’re protected from you. Haven’t you noticed how none of the spells
you put on them are binding? You can’t torture them. You can’t touch
them. You don’t learn from your mistakes, Riddle, do you?”
—Harry Potter and the Deathly Hallows