I think pretty much everybody expects galatian vets to be a trial run for 'over the shoulder' attacks becoming a standard thing going forward. If that indeed comes to pass, then I could see 10 man blood knight units being a thing. Probably not as much of a thing as 2x5 models, which is just more versatile, especially since our biggest buffs are for summonable units only so we don't get that much extra punch out of reinforcing non-summonable units.
Sadly, I don't think 10 model black knights will ever be a thing under the current battletome. They just don't punch hard enough to make punching with twice as many a big deal, even with summonable buffs. Most of what little impact they do have comes from the mortal wounds on the charge, which is per unit not per model and thus specifically discourages reinforcement compared to MSU.
We'll see, though. That's all just speculation about future rule changes.