I disagree that it needs to be translated "verbatim" to preserve the authenticity of the original. To translate "verbatim" is a contradiction. Strictly speaking it isn't a quote, or it would be in Italian. It's a translated quote.
Think of this way: would someone edit "Huck Finn" when quoting it?
That's not a translation, when you quote a work.
Your goal in translating into English is to preserve the meaning of the words. If, at the time of writing, the Italian word for "Negro" was an acceptable and non-offensive way to describe black people, then the words "black people" (or similar, like "dark-skinned people") would be the appropriate translation.
There is plenty of precedent to this. For example, if you quote from the Bible, you aren't really quoting it (unless you are writing in Hebrew) and over the years translators have changed the translated versions to better match what the original writing really said.
A possible option would be to add a footnote (eg. "In the original Italian, the word for "Negro" was used"). This might help to emphasise the point that not only did people think blacks were lazy, but they referred to them in a way which is now considered derogatory.