so what happens to any Block Mods I played before the Special Block? Are they now invalidated? Or are you perhaps not allowed to play a Special Blocks once you have played a Block Mod?
No - You may play these cards in either order and one type does not limit the play of the other type later. Nor does one card invalidate the play of a prior card.
The issue of timing becomes clear if you look at the rules for sets on page 10 of the Orc Wars rules:
"When you play cards during an Attack Sequence, you may play a single card or a set of cards. A set may contain Bases and/or Mods only. For example, you could play a Base Attack and two Attack Mods as a set, or two Base Blocks and a Block Mod. Since an Attack Sequence may only contain one Base Attack, you may not play multiple Base Attacks as part of a set. You may not play Special Attacks, Special Blocks, or Miscellaneous cards as part of a set. Such cards must always be played singly."
In basic HeroCard you may play only 1 'Exlusive' action card on your turn. This generally means that you will only be able to play one Base Attack card. This can be played in a set with 1 or more Attack Mods in one throw. Blocks on the other hand do work differently because both Base Blocks and Block Mods are 'Fast' actions. This means that in one play you could throw down 2 Base blocks and a Block Mod as on set.
Again you may not play any Special Attack of Special Block cards in a set with any other cards.
Peter - The Orc King