It has to be decrypted at some point to be viewable, so if you have system administrator access to the OS and device your browser is running on, someone will figure out how to identify which pixels and which audio bits are ads and block them from making it to the frame buffer (and whatever the equivalent is for a sound card). Where there's a will there's a way. There has just not been a need for that yet.

It's just an arms race, and the makers of uBlock Origin will need to level up and figure out how to do some serious computer vision and heuristic anomaly detection.