Unlike other jobs, Blue Mage must select which spells and job traits to go into battle with via the magic menu → set spells. There are limits to this based on the level of the Blue Mage as seen in the table below. Blue Magic Set Points Lvl. Points Spell Set Limit 1~10 10 6 ...
If memory is correct, that is a one hit MA that is similar to a Gekko that takes STR modifier (I think it is), so a SA that crits that spell would be crazy. However, without TA, you might have trouble surviving. So most of the time you wouldn’t even do something this crazy ...