Is Roger Taney's assertion that "members of the negro African race" were not actually citizens of the United States a fact or opinion?

That is an opinion.

Explanation:

A fact is that which can be scientifically proven and which allows itself to be submitted to proof, by some kind of experiment, resource or document that guarantees that the fact is correct and that it really exists.

An opinion on the other hand is a private and personal view of a fact, which may or may not be correct and depends entirely on an individual's thinking.

When Roger Taney claimed that "members of the black African race" were not really citizens of the United States, he expressed his personal and private impression on a particular subject. he expressed his opinion.