It is possible that we would like to cancel merge-queue after setting a label. (Like, finding some last minutes improvements / fixes). It worked before in bugzilla: after putting cq+, then if we put cq-, the commit queue was canceled.
The merging-blocked label, as advertised, blocks merging. It is the equivalent of cq-. It's not the only thing that can block merging, updating a PR also prevents merging.