There is a spell in Complete Arcane (and Spell Compendium) called Blackfire, that unleashes the eponymous blackfire energy. But what is blackfire itself, apart from this one spell? What are its properties, what is it comprised of, what is it capable of?
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In D&D 3.5, it's just a spell.
The spell has a really cool name, but it's just a spell. It isn't referencing a concept that exists outside the spell description the way, say, touch of Juiblex (Book of Vile Darkness) and slime wave (Spell Compendium) reference Green Slime or the black sand spell (Sandstorm) references the Black Sand environmental hazard (also Sandstorm).
So what we have on Blackfire is right there in the spell description. It's black. It's cold. It spreads. It burns away the life source of those who get caught on "fire". If you want to create a defined environmental hazard out of it (and if you like the concept, you should!), that's what you have to start with.
You may want to involve the Black Flame Zealot prestige class (Complete Divine). The name certainly seems to thematically fit, but I see little that indicates a connection between the class and the spell.
For the record, I would love to be wrong about this.