Your most powerful combo for Black Mage would be: (Must have Astra Fire III, Firestarter proc) Use Quelling Strikes > Raging Strikes > Flare > Fire 3 > Convert > Swiftcast > Flare > Tranpose. This combo will do between 3.5k to 5k damage in total depend on critical.

Also you should use Flare at the end of your rotation when you about to run out on mp then just Tranpose. Learn to use Flare people it's your most powerful spell and can critical for more than 2k damage.