it is euphonique, I think. meaning that it is used to make it sound good. I don't think it means anything. Like the T in "y a-t-il".
Only for the exercise in which you are translating from English to French. The Listen to French would require that you pay attention to the difference in sound.
Is this referring to skin colour or is it describing an object or can it be both?
From my limited experience: Yes, 'noir' is used for the ethnicity too, but the word carries a lot of complicated history and baggage, so you should avoid it.
quelle genre de prononciation est-de vous faite la liaison entre "l'une d'elles est noire" (d'elles est) this is absolutely wrong, where did you hear people speak like that???????
Literal translations are not always good translations. In English we would not use "the" there.