You can fix this in your courses by clicking "Yes" to "Open in New Window" when attaching files to Blackboard. This setting will force the browser to open the file correctly.
This is a case where Blackboard is caught between Apple and Adobe arguing about who is supposed to fix this problem. However, Blackboard is looking into a way to get around this. I'll let you know if they come up with something. In the meantime, just say "Yes" to "Open in New Window."
Students can view PDF files on Macs by using the Chrome browser which is a free download from http://www.google.com/chrome.