I can't add any blitz plays to custom defensive playbooks, neither call one during games from default playbook.

Looked up some blitz plays from plays.xml (4-3 edge blitz, crash middle, dogs man etc.) but I can't find any of them in game . The "type" selector is greyed out while blitzes are a distinct play type in the .xml structure (off/def/blitz). Maybe it has something to do with it.

I was using default playbook too and I can't find any blitzes. Are they in the coaches playbooks?

When a blitz is called based on the blitz settings it will select a blitz for you. The blitzes are not supposed to be in the custom playbook editor. If you want them there to select fom you can copy them in the xml and insert them into the default playbook. (remember to let them remain in the blitzplaybook as well).

But how do you call a blitz during games? Settings shouldn't matter when manually calling plays.

I'm so confused by that answer, and i don't know how to move folders and files and stuff around.

When calling plays you can't force a blitz. The blitzing is determined by your strategy and the team shifts into those plays.

We don't expose the blitz plays in the XML directly. We may be able to change the UI up a bit to allow you to check a checkbox for blitzing.

As a side note in the plays mod I created the blitzes that are provided in-game are there as callable plays and show up with the play art.