I have noticed a number of thaumaturge enemies, mainly Garlean conscripts and the Vault Priest near the end of the Vault, have some "Dark Fire" abilities. For example, "Dark Fire III/IV" is kinda like Flare but also inflicts burn effects. These are not the same arts as the standard black magic spells as taught by THM guild/BLM job crystal, so what exactly are those "Dark" spells supposed to? And how are they different from the "Void" magic used by imps/succubi/etc.?

I thought at first they were magitek, until I saw the Vault priests using it.