The collar seems to interfere with magic generation somewhere far upstream of getting anywhere near that far, blocking her before Opalyn can really generate or call forth the magic; the item does not try to let her generate it and then block it! This makes sense; if magic items could generate power on anything like Orphan's scale, then Orphan would not have been able to lawnmower over wealthy enemies.
It's like being drunk, maybe, in a very selective way. Like your motor neurons are in their own personal REM-stage dream, where every time they try to be sensible they just end up feeding the pillowcases back to the crocodiles again. The impulse to cast a spell is not going nowhere, but it sure is not going into generating the magic that could actually feed into that spell.