Erinna still suspected that Evil Dragon Sword Ancestor had signed a Dragon Knight contract with Black Dragon, and even wondered if they had known each other a long time ago.
A big reason why he was called Evil Dragon Sword Ancestor was likely because he was Black Dragon's knight.
Erinna guessed this before today, she had never interacted with Evil Dragon Sword Ancestor, and was unclear about the relationship between Evil Dragon Sword Ancestor and Black Dragon — she could only speculate after seeing how Evil Dragon Sword Ancestor and Black Dragon interacted.
Even if Evil Dragon Sword Ancestor hadn't signed a Dragon Knight contract with Black Dragon, their way of interacting was probably similar to hers with Erinna; they were friends who, when necessary, could also be comrades-in-arms.
Unless necessary, one would not actively become a knight of Pureblood Dragons.
"Do you want to become a Dragon Knight?"
"Yes."