Blackfire is a manifestation of Hylomorphic Sorcery, which is unique to Warrior Mages.
Then again we have this feral magic stuff which has upended magic theory in general, so maybe Bards can use it now, who knows?
Though for real old Blackfire was broken and and Pyre is already pretty OP, giving Bards access to cyclical AOE Blackfire would be more broken than Fire Rain, Naga, and Universal Solvent combined. Maybe some cool fire metamagic that is not Blackfire specifically?