Hi, Black Mage Main chiming in.

You're doing it wrong.

Basically, until you get your 'III" spells, you:

Fire 1 until empty, transpose, thunder, fill with Blizz 1, transpose to fire, repeat.

When you get your III spells, transpose is an emergency "Damn I don't have enough to Blizz 3."

Fire 1 to near empty. Blizz 3. Thunder. Fire 3 at full, Fire 1 to near empty. Repeat.

Sorry, there is no other 'good' way of playing Black mage, because the way I listed is still horribly sub-optimal, and that's 85% of playing pre-60 Black Mage.