What do ad blockers actually block?
Lists. uBlock Origin, Brave, AdGuard and Safari blockers ship EasyPrivacy and similar lists of hostnames and paths. A script loaded from googletagmanager.com, plausible.io or hotjar.com matches a list entry and never runs, so the visit is never counted. A script loaded from your own domain on an ordinary path is not on any list. That is the whole difference between blockable and first-party above.
How to count blocked visitors
Serve the analytics script and its beacons from a subdomain of your own site. Statsy does this with one CNAME record: point s.yoursite.com at Statsy, save the hostname, and the snippet changes to load from there. The 2 KB script is the same; only the address changes, and blockers treat it as part of your site.
What this check cannot see
Scripts added after the page loads by a tag manager, a consent tool or an app bundle, and pages that refuse bots. When nothing is found, the estimate assumes a typical third-party script. Blocker rates are averages; your mix of devices and countries moves them.