I'll start with common sense: they have a lot of resources deployed right now to retrieve the black boxes, which they would be unlikely to do if they knew they were in an area which made it impossible from an engineering standpoint.
Alternatively: James Cameron, Marianas Trench. Here's his explanation from today on how the black boxes would likely be retrieved. He points out that his submersibles can go twice as deep.
http://www.thedailybeast.com/articl...limate-change-leonardo-dicaprio-and-more.html
I guess let us know why Cameron is wrong, or why it's impossible or outside of the bluefin's capabilities? I will follow any science-related links you provide.