Translation:Her hair is both black and beautiful.
I understand, but the course just in its beta. If you would like, I think you can still sign up to contribute https://incubator.duolingo.com/courses/zh-CN/en/status
This English sentence is just so awkward that I can't really imagine ever saying anything like this, neither these words nor this structure. The closest thing I could think of that would actually be idiomatic is 'She has beautiful black hair' but that as the English example would not help anyone to guess 又黑又好看。But that should certainly be accepted as an English translation.
好看 and 漂亮 differ somewhat subtly. From my experience, 好看 is gender-neutral and can really only describe people (it literally means "good to look at" or "easy to look at"); 漂亮 is considered to be somewhat implicitly feminine, is also used to describe both people and inanimate objects, including hair and clothing.
The audio prompt for this sentence makes it unclear that the pronoun should be 她, as one might conceivably comment that a man or animal's hair is black and beautiful. To my knowledge, 好看 isn't a gender-specific adjective, and there's really no reason for this sentence to be gendered (or human-specific, I suppose).
Also, echoing previous comments, that the translation is not an idiomatic one. The 又A又B construction for adjectives doesn't really mean "both A and B", but rather that a noun is "A and B"; it describes two adjective qualities that occur together rather than imply that the two adjectives are related. 又黑又好看 doesn't necessarily mean that hair color and aesthetic appeal are related.