And immediately after posting that I figured out the solution. I’ll write it here in case Googling led you here.
the filterBy('template) function is defined by what template the page will end up as, and not what kind of blueprint you used to make it. So, if you have the content file as act_ballroom.txt, but you haven’t yet build a act_ballroom.php template in your templates folder, this will come through as default and therefore your filter won’t work as intended.
TLDR: Make sure you have a template matching your blueprint to use filterBy(template)