Black blades are going to be important in the sense they are Zoro's EoS Powerup, a power he shares with only a handful of characters as of now, and It's likey going to be the very peak of CoA (maybe alongside other conditions and even some CoC, but mostly CoA).
That's it. They don't have the relevance of peak CoC or Awakening and I don't think they'll have a specific usage like AdvCoC on immortals had, for example.
However, they are going to be addressed and hyped as a peak of a type of haki and the final Power up of one of the mugis.