For some (in my opinion pants-on-head-retarded) reason the maintainers of the chromium package in debian have chosen to disallow loading of extensions from the chrome store. They claim that this will pave the way towards “distro-packaged” extensions, but as of yet I haven’t seen a single one in the repos, not even ublock. What’s worse, the only notice you get is if you upgrade from a version before this “fix” was applied. Users on the mailing list ain’t happy either.
How to unfuck this
The best way I have found is to edit the .desktop file at /usr/share/applications/chromium.desktop and replace the line Exec=/usr/bin/chromium %U with Exec=/usr/bin/chromium --enable-remote-extensions %U.
I certainly hope the maintainers come to their senses, as doing this after every update is ridicilously inconvenient and it breaks the chromium ecosystem.