I am 100% with you on this.
However,
If its monster, than you in the initiative stack, monster attacks you, block goes up, then next action you lose the block.
If its you than the monsters, you do your thing, monsters attack, block goes up, then the block is valid through the entire stack, even against friendly fire (common occurrence) assuming the block is not annihilated.
You did make an excellent point though. It doesn't say in the book, because it was already said in the evocation block, thus, a block, once generated, lasts one round or until destroyed/cancelled/replaced.