Blom made many right decisions and few wrong ones, but those were important.
Blom together with Kircher and Rocchi were the only non-Elite referees in this stage of the Europa league, which means that UEFA has some plans with them. Blom did not disappoint but he also did not delight. When I read his bio I see that he saw his fitness as his strongest point, but in this match especially in the second half, he was far away from the action, maybe he trusted his assistants, but his fitness is a big question for me. Also, he added that a lack of concentration is his weakest point and there he was right. A few times he stopped the match when he could have let the advantage, and in the eleventh minute Werder could have a chance on goal if Blom hadn't used his whistle. His valuation of fouls were mostly o.k., penalty for Werder also well spotted, and all cards were showed rightly. But at the end he must have thought that one Werder player deserved yellow for protesting and he pulled out his card, but then he surprisingly didn't show that player a yellow card, but put it back in his pocket. A referee should never do that.
They were mostly good, but the second assistant didn't see that Villa was in offside position at least half a meter when he scored the fourth goal. That situation was not hard to judge, even both assistants had many closer calls during the match, and it is bad that a decision like this could decides which team will go to next round.
These teams never seem to have heard the word defense. After this match the audience is happy and coaches have lot to say in the locker room.