How would you explain it with coordinate covalent bond? I think any quirky thing is tagged CCV?
In this case anyhow the extra e in C is from the environment? Yeah, I think it's from the environment when CN comes into contact with other substances that donate e. Simply like how you have H+ and Cl-. CN- is the same. It gains e from anything like H, Na, K atoms, etc. Remember CN- doesn't exist on its own. It either covalently or ionically bonds with other 'things'. Yes, that's easy.