The answer from an Imm being something along the lines of that you're pretty safe going off the AC when it comes to prot against blackjack- reason being that after they standardized the eq the AC should generally reflect the quality/material/coverage of the headgear in question anyways.
I don't recall them definitively answering the question directly on which property the protection checks against, however.
Wish I could find the quote- I did a bit of searching but that on the officials is very painful and I gave up.