[The Saucepan Man]

Beorn is described as a sorceror and had (at least) skin changing abilities.

I did not include Beorn, because he is not evidence of the existence of "learned magic". I agree that his magical powers were probably inherent. I am not saying that all who are described as sorcerors necessarily acquire their magic through study.

The Mouth of Sauron was a great Black Numenorean sorcerer.

Yes, you are right Salocin. I should have included the Mouth of Sauron. But, he falls into the same category as the pre-Nazgul Nine. It is, as you say, unclear whether he became a sorcerer under Sauron or went into Sauron's service because he was a sorcerer. Accordingly, he neither proves nor disproves my theory.