I want to clarify:

When having a ship with "bording" in the staging area. I do not engage that ship optionaly (becvause no action window before in that phase), instead playing Revealed in Wrath on that ship (as long as it has no printed "immune to player card effects") an dthen engage that ship by engagement checks: the textblox is blex exepct for traits. So, no boarding, right?!?

Indeed, no boarding.

Boarding X is a keyword (like Ranged or Archery X) and not a trait (like Ship or Corsair). Thus it is blanked along with the rest of the non-traits part of the textbox when you use Revealed in Wrath. And, while so blanked, ships will not have a Boarding effect when engaged.