KMST 1.2.472 - Wind Archer (II), BGMs

Is that last quote supposed to signify that this is the future if the black mage acquires the three trancendences?

I don't think so. Black Mage has always had the quite powerful ability to give eternal life to those who sold their souls to him, even though he's not the Transcendence of Life himself (Alice, the World Tree, is).

Just look at Von Leon.. he sold his soul to Black Mage in exchange for eternal life. He was given it.. but he was corrupted by black magic and transformed to that huge lion monster we can see now.

It seems that he (Black Mage) was able to master black magic in such a way that allowed him to do so. Obviously, though, we all know black magic's side effects..

I'm prone to think that it was black magic which corrupted Black Mage's soul and made him become what he is now.