I have it in Astrid's backstory that she tried to practice thaumaturgy once but utterly failed at it... with the exception of ice magic. I figured that if conjurers are adept with water, why couldn't they freeze it as well? Also, if we're including mechanics in this... Blizzard II is a cross-class spell CNJ/WHM can use, so knowing that, I didn't think it was too much of a stretch. That's just my two cents.
