IIRC, the designers have said the oath-breaker paladin is intended to fulfill the same function as a blackguard. However, the fit isn't 100% perfect (at least based on the name; the DMG isn't out yet so I can't comment on the specifics). A blackguard isn't really just a "fallen" paladin. They're the evil version of a classic paladin.
Furthermore, 5e, like 4e, has eschewed the typical LG restriction on alignment for paladins (the PHB arguably encourages it with the fluff, but there's no mechanical requirements and the text outright says a paladin only needs to follow their code, rather than specific alignment). So arguably a blackguard could just be an evil paladin (presumably with the Oath of Vengeance).