When using frozen blueberries can they be used frozen or must they be defrosted?

Use them frozen for sure, but you might want to bake an extra minute or so to compensate for the cold start.

Agree with Ttrockwood- like most fruit, when blueberries freeze a lot of the cells burst and you get a soupy mess when they thaw. Baking with them frozen isn't ideal- they will produce small cold spots while baking and will still release a good deal of moisture when they bake, but these aren't really serious problems and using them frozen is much preferred.

Definitely add them still frozen! Otherwise they turn all the batter of the muffin an odd blue color... just add them in frozen and stir minimally