If {ordric, master tactician} and three other creatures attack and I declare that no opponents creatures block and the opponent cast a flash creature is that creature able to block?
Once the declare blockers step comes, and you declare that no creatures block, it is too late to flash in a creature. If your opponent would have flashed it in before the declare blockers step, it would not have even mattered because you still choose what blocks/does not block regardless of whether or not your opponent gets new creatures in between combat steps.
RESOLVED
Yes that is correct. The key being, you choose which creatures block, not which creatures don't block.
Opponent flashes in creature after. It will not be on list of approved blockers