Oh, yeah, you're right. I misunderstood what you were asking. That field doesn't support icons yet. I should be able to make that change in the next update.
Edit
Maybe I'm going crazy, but the default status menu, nor yanfly's status menu include the ability for parameters like HP, Attack, etc to have icons in their names. If you're using another plugin to make this change in your menus for those base parameters, I might be better served making a minor extension to it for the block % text, rather than trying to make this specifically for the block % text.