FFXIV Thaumaturge Guide: Black mage

The Thaumaturge's Guild is in Ul'dah - Steps of Nald. If you choose thaumaturge as your first class, you will start off in the city of Ul'dah. At level 30, thaumaturges may specialize into black mage.

The following crafting classes can create items that are useful to the thaumaturge:
Equipment
Carpenter — Wooden shields.
Weaver — Cloth armor.
Goldsmith — Staves, scepters, earrings, bracelets, necklaces and rings.
Your basic rotation as a Thaumaturge should be to start each fight with Thunder, then use Fire spells until you're low on MP. At this point, use Transpose to switch to Umbral Ice, then cast Blizzard spells until you have full MP again. Once that happens, Transpose again to switch to Astral Fire, then repeat the process. If Thunder runs out and the enemy still has a good amount of health remaining, use another Thunder spell right after Transposing to Umbral Ice (so you're regaining MP while casting Thunder).

When you make the transition to Black Mage, a few things change. At level 34 you gain Fire III, and at level 38 you gain Blizzard III. While Fire II and Blizzard II are area of effect spells (AoE), the third version of these spells grants full stacks of Umbral Ice or Astral Fire. The more stacks you have, the more damage you inflict or the faster your MP replenishes. Once you have both spells, you no longer use Transpose unless you end a fight in Astral Fire, in which case you Transpose to switch to Umbral Ice and replenish your MP.

With both Fire III and Blizzard III, your rotation changes to: Thunder (the length of the fight determines which Thunder spell to use, as each spell lasts a different amount of time) > Fire III > Fire (until your MP is low) > Blizzard III > Thunder (if it has worn off) > Repeat. The only change to this comes at level 44 when you learn the Firestarter trait. This grants a 30 percent chance that a successful Fire cast will make your next Fire III require no MP and cast instantly. When this happens, instead of casting another Fire spell or Blizzard III, you cast Fire III.

When you reach level 50, Black Mage learns the spell Flare. This spell requires a minimum of 266 MP and consumes all of your MP, but inflicts more damage than any other spell. It's also an AoE spell, meaning it hits multiple enemies in close proximity. It's typically used at the end of a rotation, before switching from Fire to Blizzard III. However, because of the casting time of Flare, it's generally more effective to use it only when hitting multiple enemies, or when you have the Convert and Swiftcast abilities available. If you leveled an Archer, Raging Strikes is also very effective when used in conjunction with Flare.

Thaumaturges' and Black Mages' Limit Breaks focus on dealing powerful damage to enemies in a circular area of effect. The damage dealt is determined by the combined weapon damage of all party members, thus, its damage is not based on the user's gear. The ability Swiftcast cannot be used to instantly cast a Limit Break. Thaumaturges and Black Mages both have the same Level 1 and Level 2 limit breaks as other Ranged Magical DPS.