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.