Hello.
Unfortunately, there is currently no way to conditionally add block rate to a battler. The notetags on database objects are parsed into traits on the objects they are found on at the time the database is loaded, meaning even if those tags were designed to take a conditional statement like that, the condition would likely be false.
Like with other xparams, such as crit % or hit %, the only way to add more of them is to put them as a trait on either a state, or equipped item. Because traits can't take a variable as their amount, the only way I envision this working is via passive states on those skills.
As mentioned, passive state bloat is a thing, and it's not ideal, but there is currently no other way to do it, as far as I'm aware.
It might be possible to patch the YEP_ExtraParamFormulas plugin to allow it to work with block rate in the same way it works with the other xparams. Similarly, adding base and "extra" block rate parameters to the block plugin could also be possible, but doing so would require me to essentially do exactly what the extraparamformulas plugin is already doing, likely with some awkward interactions between the two of them.
~Ramza