I can't seem to find the rules in YS or a thread relevant here that tells me exactly how a block stored on a magic item is activated.
So, Tony the Sniper and his twin Timmy get the drop on you; you have no idea they are there. They fire simultaneously, both rolling 5's to attack; the sound of their guns will reach you after the bullets do. Surprised, your athletics is treated as 0. You have an enchanted duster with a block of 10, but it only has one use. What happens?
Most of the forums seem to say that because you've got this block ready, both bullets will bounce of the block, making it a great defensive item as it stops this sneak attack cold.
Everyone agree?
But, what happens next? Did it activate automatically, or does it cost you your action this exchange? When can you act? Should the sneak attacks be considered part of a previous exchange, and initiative called for after the shots hit?
The only answer that I rule out immediately is that you put a condition into the spell that says "if I get attacked by surprise", because all other parts of the book (in particular, the part on wards) indicate that magic can't be that discerning.