This is a known issue in the newest version of the Blackboard text editor.
Control X- Cut
Control C- Copy
Control V- Paste
Command X- Cut
Command C- Copy
Command V- Paste
Also: Firefox 22 has broken the ability to copy and paste in Blackboard via keyboard shortcuts on both Mac and PC. Mozilla is aware of the problem and will fix the problem in their next release- Firefox 23.
Workaround: Use a different browser, Chrome on PC or Safari on Mac.