Thanks for the response. The skill is based on one by Yanfly which basically sets the success rate to 0 if the incoming skill is on the list.
I tried adding an if/else at the beginning of the <Custom Select Effect> checking for a parry/block like you suggested and then activating the skill if false, but my JS skills are pretty basic and not 100% I did it correct.
I don't think the block/parry is the issue though. If I turn the Parry plug-in OFF, the skill still activates with the Block plug-in ON. Also when Parry is ON, even though none of my characters are capable of Parrying the skill doesn't trigger.