I thought just her denying to cast the spell was sneaky – But I agree that Blue definitely looks like her mouth is moving. And it doesnt look like an intake of breath or something, multiple times her lips part like she is speaking under her breath.
So was Blue helping the Darling boys get into Storybrooke by stalling, or did she have another motive and the result just happened to be the boys making it to the town line in time?