the 'Blocked' checkbox under the "User status that can be referenced" field setting does not take.
That's most probably because the associated value is 0.
There was a long standing core issue about that, and I thought this was finally fixed. I can't seem to find it right now, though...
|FAILED: [[SimpleTest]]: [MySQL] Unable to apply patch references_allow_referencing_blocked_users-1038102-11_0.patch. Unable to apply patch. See the log in the details link for more information.|
|PASSED: [[SimpleTest]]: [MySQL] 40 pass(es).|
|PASSED: [[SimpleTest]]: [MySQL] 0 pass(es).|