The most useful monster.

Perhaps you are mistaken; the description states that fiery breath is an ability to strike multiple opponents, and nothing is mentioned about a fire attack:(

I would humbly point out that the breath is indeed fiery and therefore is a fire spell with all the consequences that entails. In particular, it is impossible to strike an ifrit, a fire elemental, or a phoenix with fiery breath. In melee combat, the Black Dragon uses a fire attack(!) that matches the attack of an ifrit, a fire elemental, and a phoenix in its properties, which means that only half of the Black Dragon's attack will affect these units.
And one more thing: you may not believe it, but the damage of the breath is equal to the damage in melee combat.