More and more often I see this word in news headlines. When I saw this word first time I thought it's quite negative as direct translation in my native language. Person who talks a lot and reveals other's secrets is not usually welcomed at all. The way how this word is formed — whistle and blower — don't imply anything good as well. So what's the true emotional tone behind this word?
A whistleblower is, in fact, someone who tells on people breaking the law or, more generally speaking, doing something wrong. So it is, in fact, a positive term used by the right side of the law. On the other hand, evildoers would call that same person a snitch, which is a very pejorative word for someone who tells on someone else.
It is implicit that the whistleblower is revealing wrongdoing within an organisation, so the tone is always positive. In the UK at least, whistleblowers are protected by law from any fear of retribution.