Maria, I have always wondered about null shields because in Magic study Moon Man says you can do magic behind the them but magic cannot pierce it. However later in Sea Glass and Spy Glass it seem that it you are encompassed by a null shield you can't do magic. For example at the end of sea glass in Hubal opal chants to herself 'break the null shield and Zitora can heal herself' and Ulrick had to take off his null shield jacket to do magic.
In Magic Study, the null shield that Moon Man refers to is a huge one that they built like a wall between them and the Daviians (it isn't very clear). In the later books, the magicians use them like bubbles to encompass people and block them from the source of magic (aka the power blanket).
Thanks :)
P.S I love your stories, they are amazing!