+X% Weapon Attack

Maybe it doesn't affect passive skills like MM boost and blessing.

From the calculations he made, 184 att was the guy's TOTAL weapon att including any blessing and MM boost (if any). The crossbow is 134 att, and he calculated that to have that damage range he'd need 184 att. So he noted that the person must have a +50 att boost from either cape, shoes, blessing, etc.

He was able to get the exact damage range by assuming that the +att% affect the whole 184 att (meaning that it does take into account blessing)

I'd find it hard to believe that the person doesn't have at least lvl 3 blessing if not higher.
However, we have no idea if he has MMboost, so we don't have a definitive answer on whether or not it would be taken into account for the +att%.... but if blessing is.. MMboost should be too, one would think.