While that's true of software set up to specifically subtract content from a server response, the code could just as well be set up to add page elements (e.g. for easier navigation) or just rearrange things. So "content customizer" or "user preference enforcer" would both be apt and less arguably pejorative.

"Blocker" implies a specific set of assumptions about a publisher having some say in how content gets presented. "UserAgent" entails a different worldview, one that the companies making browsers seem increasingly unwilling to defend against competing interests.