I’ve never understood why blockers “block” ads in a way detectable by the site. Just accept it as if you were going to show it, but throw it away.
With the video ads on YouTube, the “missing time” that would have been present had you watched the commercial is a huge giveaway. And other detection methods involving protocol acknowledgements and probably other industry-secret methods are used. One more elaborate strategy is to utilize a service that responds normally to the ads, but passes ad-free content on to you. Some VPNs and speciality sites offer this.