:: Perhaps in cases such as these, it can count as long as the review is primarily focused on the art? ::

That sounds like a good way to handle it. If you notice that the white author is doing something squicky, it is appropriate to call that out, but the POC illustrator should still remain the main focus of the review. Also do make it clear that the author is not POC, and that you're reviewing this book on the basis of the illustrator.