Why does Rosa expect Rydia to be able to easily handle Black Magic?

[*Laurelindo*]

Well, technically speaking it has at least proven that Summoners are unlocked by leveling up a Black Mage.
Still feels like slightly different things though, Black Magic is more about controlling the elements, while Summoning is about calling entities, and Summoners don't directly use magic, they have the summoned monsters use magic for them.
But maybe I'm looking too much into things.