Does anyone know how block rate works?

[Shurrikhan]

blockrate used to scale with str. dunno if it still does.

It does not. Parry and Block percentiles were both decoupled from Strength at the start of Heavensward.

Addtionally, shield block vs. strength balance has been equalized across all Stormblood shields. There is no longer a reason or way to choose between block frequency and Shelltron strength.

Rather, the chance rating and resultant block percentage has simply tapered over the course of 61-70. Unless PLD is slated to gain relative power over time, it's quite possible that each is also capped to a fixed percentage by 70, and will not improve over the course of additional tiers.