Editors have told me that their sales departments say black covers don't sell. Sales reps have told me that many of their accounts won't take books with black covers. Booksellers have told me that they can't give away YAs with black covers. Authors have told me that their books with black covers are frequently not shelved in the same part of the library as other YA—they're exiled to the Urban Fiction section—and many bookshops simply don't stock them at all.
Clearly, as Larbalestier says, "all of us—writers, editors, designers, sales reps, booksellers, reviewers, readers, and parents of readers—will have to do better."