- (no need to enter any values for any preferences) go to Special:GlobalPreferences and enable email-blacklist globally;
- go back to Special:Preferences and check the "Set a local exception for this global preference." checkbox;
- the associated field should become enabled;
- try entering a username: the autocomplete will work, but all of the options will be greyed out. There is sometimes also a bug where as soon as the dropdown list opens, the page is scrolled back to the top.
If you save the preferences form, the field works correctly (i.e. because it's already enabled when the page loads).
Is this something strange about how the UsersMultiselectWidget is disabled/enabled? It seems that it's being infused correctly, because it does actually become enabled and does work. The code in question is around ext.GlobalPreferences.local.js line 19.