I have checked and made sure that both your model of Mac computers play the game without an issue. We have several users who play with those Macs and perhaps some of them have an issue with a single map itself but not the black screen of doom.
If you haven't yet make sure to verify your cache(instructions below) to see if that helps and check if any other programs are possibly interfering. Like an anti-virus or gaming program.
1: Open your steam library
2: Right click Dungeon Defenders (
http://i.imgur.com/UyPCa.jpg)
3: Click properties (
http://i.imgur.com/JvLz2.jpg)
4: Click local files (
http://i.imgur.com/TO5wC.png)
5: Click verify integrity of game cache (
http://i.imgur.com/IZDGD.png)
If those don't help it's hard to give you a reason why you're seeing the issue since we got confirmation that those models or quite similar work without an issue. We are also unable to replicate the Mac issue on some in-house testing. We're still releasing some more updates this week for Mac users and perhaps one of those will help.