After another review of the episode, it seems that celestia and Luna have something else in store for twilight.
I think they want to make her an elemental pony like they are. "twilight sparkle"? kinda sounds mystical or starry. I dont know what kind of element she may be other than magic.
we have candence who is love and peace... then celestia of the sun, luna of the moon ... blue blood is just a conceded unicorn that probably has not reached that level of maturity yet,but very powerful in some way.