What is an "unsafe airline"?
There is no international blacklist of unsafe airlines. An airline can only exist if at least one country believes it is safe. Those regional blacklists are about the best you'll get. 

In a formal sense, calling an airline "safe" is generally avoided. A regulator would use a term like "meets safety standards". This might be a small distinction but it helps make it clear that no airline flight is guaranteed to be safe. So the closest legal definition of a safe airline is one that has been certified as meeting safety standards by an ICAO compliant country. However this is not that helpful as different countries have different standards, as proven by the EU and FAA blacklists. 

So to get an international comparison for airline safety you need to look at informal sources, which is subject to opinion. But this opinion can be formed by combining multiple official sources. A good example of how this can be done is on https://www.airlineratings.com/safety-rating-criteria/. This criteria is informal and doesn't consider the the most important behind the scenes information, but it is probably the best safety indicator we have.