Seems like it was the power-button looking icon found in the upper left part of the extension dropdown menu when on an actual page.
I expected this to be a part of the extension settings / options, but ah well.
I expected this to be a part of the extension settings / options, but ah well.
The "all" cell (top left corner of the matrix) is a "soft" allow all, i.e. it allows everything except those hostnames and types of request which are not specifically blacklisted.