If an enemy has swiftness and 3 ice attack in a blue city then how many normal block points would you need to successfully block the attack? What about fire block points?
While a shortcut is to think about how many normal block points one needs, it's best to follow the logic of when a block needs to be doubled and when a block is halved.
The enemy has 3 ice attack. Since the enemy is in the Blue City, you consider that to be 5 ice attack.
The enemy has swiftness. Thus, the block needed is doubled. So now you need 10 block.
Only fire block or coldfire blocks are efficient. Any other block is inefficient and needs to be cut in half.
So you need 10 block. If you only have physical block, you would need 20, since it's inefficient and would be cut in half.
But what if you have a mix of fire block and physical blocks? Simply cut the physical blocks in half. So 4 fire block + 12 physical block (cut in half to 6) would get you the 10 block needed.
What if you have guardsmen who has 4 physical block that are doubled against an enemy with swiftness (using Lost Legion version)? Now you have 8 physical block, which is cut in half due to inefficiency, which means you have 4 of the needed 10 block.
All these situations are clear so long as you follow the actual logic and not shortcut.