Why is a pixel blocked and what does that do?

Pixels get blocked for a simple reason, since its a javascript code loaded from a standard url, privacy technologies can simply block the url and thats its. No more pixel. Case in point:

The url above is the facebook pixel code. Its always the same url, for everyone. So an ad blocker? They just don't let anything load from that url. No code loaded, no data tracked, nothing for facebook to pickup.

The result of this is that:

So all in all? You're flying blind through the marketing world.