If attacker rolls first and defender second. How does the attacker block damage?

I assume you mean with the expansion rules, if so:

If the defender dealt damage with a card or there is damage left over from a previous round.

This is the disadvantage of being the attacker. You'll notice there are tactics card that reverse the order of rolling dice as well -- I think that's to flip this discrepancy .

This has been discussed several times.

Players have tactic cards to block damage. Players have tactic cards to swap play order for combat.

On top of that, with the new reroll aspect of combat, this just means higher potential damage from the first player. You'd be rerolling those blocks to get hits and crits.

Blocks make up 1 of 6 die faces for red/black. They don't exist on green dice.

In the grand scheme of things, the number of times this will matter will be slim. Beyond all the issues I listed above, most of the combat that happens tends to be pretty one sided.

Previously the attacker had a slight advantage. I can't think of a time when that advantage would have tipped the balance of a battle or the ultimate resolution of the game. I have a feeling that this advantage will be as minor.