Question about class upgrades

[Shisou]

The misconception you're running into here is that BLM is a Class, such as THM. Instead, BLM is a Job within that Class... think of it like a specialization. Once you've reached the requisite levels in THM and ARC and completed the appropriate THM Quest, you obtain an item called a Soul Stone that makes you a BLM when equipped. You retain your THM abilities, while gaining the new ones of your Job as you level them. Equipping this item also restricts what cross-Class abilities you can select from your other Classes -- BLM, for instance, is restricted to ARC and ACN for cross-class, whereas THM can use several other classes' abilities.

Hope this helps answer your question!

EDIT: To clarify, your Job gives you additional Skills, but is not leveled independently. When you hit THM level 50, for example, you gain Aetheric Manipulation and also gain Flare while the BLM Soul Stone is equipped.