Ultimately, nobody really, genuinely holds the opinion that an actor can be of any appearance for any role. There will always be exceptions that will break their immersion, because unlike with theatre there is the expectation that the movie is WYSIWYG.
To give an extreme example, would you be completely undisturbed if the main character in 12 years a Slave was portrayed by John Malkovich? Or if a Jewish camp inmate in Schindler's list was portrayed as a black person? I don't think either example would sit well with the vast majority of people, no matter how racist or not racist they might be.
That's why pointing out that black people did exist in Italy is correct but beaide the point. The real debate is around the acceptability of certain expectations, and what determines the criteria for this acceptability.