Clan Titan

do you make 120 damage for every weapon/slield/armory(etc.) that have blast, or do you just make 120 damage undependent on how many different rares with blast you have?

I don't think the effect can stack...

Blast essentially works like this:

Hero: I am attacking!
Game: Ok, do you have anything with Blast?
Hero: Yes!
Game: Did you do less than 120 damage?
Hero: Yes!
Game: Ok, you do 120 damage!
Hero: YES!
Monster: ARGH

lol that's an interesting way of explaining it.
But I remember a long time ago you could stack the effects of blast powers (I used to kill dragons by stacking 70 or 80 powders in one round). I'm not sure if that's still possible today.

so... your´e saying that blast is only good in lousy weapons?

Pretty much.

Blast Powders work differently than the weapon ability Blast. The weapon ability basically calls "blast powder x 3" when it does damage.