They're segregated from the rest of the books when you go into bookstores. I think it's stupid, but alas, you have an AA section where all and any books by blacks are stored, regardless of it's subect. Same at many libraries. You have keep looking for them. Not all of them portray blacks in a negative light. There are many thata have the same high school drama you see in books with white MCs, except teh black books will of course have blacks that go through thsoe same dramas - boys, backstabbing girlfriends, bullying, etc. Universal things taht teens go through and like ot read about. Can't correct my typos because my coputer is slow, so sorry for them. I agree, too that you don't find that many books with black characters and when you do it's whitewashed on the cover to make it appeal more mainstream - cheack out the original cover for a book called Liar by a white author but the main charactter is black.
ADDING: If you want to find these books then you have to know where to look for them. Look for websites that are dedicated to black teen readers and you'll probably find a lot of book recs for YA. There are websites and blogs that are dedicated to helping people find books by black authors since they;'re so hard to find. If you like fantasy, there are some black fantasy authors who are making a name for themselves in the fantasy field (A hundred thousand kindgoms by n k jeminsin, for example). There's also Octavia Butler. If you want younger, more current fantasy authosr then you can check out: Nnedi Okorafor, she hwas writetn some YA fantasy novels and I think she's won some awards. Jeminsin has been nominated for some of the more prestigious fantasy awards.
If you don't care about fatnasy and just want urban fiction check on Avon romance, Harliquin, and Kinsington websites and see what kind of books they have under their AA section for teens. They're all publishers.
If yoiu're looking for a more philisophical answer rather than recommendations, then publishing has had this mindset for decades taht the majority of white readership will not buy a book with a black person on the cover. I don't know if this is factually proven by studies and sales or if it's something they're assuming. Back in the 90s, many black authors had to prove themselves before a publisher would take a chance on their books - they had to self-publish and get the sales and then a publisher would take their books sseriously. Zane is one and there are plenty of others who have gone on to be best sellers. One publishers saw that there was a market for books with black characters (because, by golly, blacks also like to read) then it became a bit easier for these types of books to sell to publishers without the author self-publishing first.
I still think there's this mindset in publishing, amongst publishers and booksellsers, that books with blacks on the cover will not sell as well as books with whites on the cove (which is still why white washing happens). The thing is that there is probably some fact to this proven in sales numbers because publisher will publish stuff that makes them money. If a book with a black character on it is selling well then yoiu would see more of it on books shelves (not just in the AA section). This has more to do with booksellers (bookstores) than it does with publishers. People don't realize how much power bookstores have (it's declinining because of ebooks). If a major bookstores doesn't like your cover then the publisher will change it to something the bookstore likes because it's teh bookstore that has the power to stock it or not and they won't stock books that they don't think will sell (titles & covers play a major role in their buying decision because it plays a major role in whether readers will pick up the book off the shelf and consider it). So there's probably some truth in it (covers with blacks on it). - it's seen as "this book isn't for me" if it has a race that's not my own on it.