What does 'cold' mean in the following context?

It means without knowing anything about it and not having a key or combination.

The word "cold" in the context of the statement means that he was dead on to cracking into the safe. He did that very fast. Good luck.

I was a college professor of education.

without knowledge of the safe or its complexities

Without any prior warning of what he is to do - he therefore is a master safecracker.