If you attack Blaise and use a card like deadeye, it triggers his adapt ability so he can become hidden. Does the hidden apply on this attack?

With Deadeye I'd say yes, it all happens during the Apply Modifiers (4) step, which comes before the actual Check Accuracy (6) step.

Interestingly, it would also work the other way round. When Blaise attacks you and you play, say, Hard to Hit in the Reroll (3) step, he could Hide himself and still get +1 Surge for the attack in the Apply Modifiers (4) step.

I'd second that; the wording would seem to indicate that the trigger is instant. So at whatever point in time the opponent plays their first command card of the round is the same moment you can put a Hidden token on a valid target.

Sort of makes Deadeye pointless in that sense, but it could also be a good tactic for your opponent to bait out your Adapt with something else earlier in the round so that when they make their intentions known with the Deadeye attack, you can't proc Adapt and make the target Hidden.

Order of operations is always the key here. Remember your 6th grade BEDMAS steps and it won't be a problem.

Deadeye must be played on or before step 4.Apply Modifiers, because it adds or removes symbols or accuracy. Hidden does its job (remove accuracy) in the same step, and is a defender ability, so it will apply in the same attack.