I have trouble testing this class because it invokes calls to JenkinsFacade that try to access the jenkins instance (which is null and leads to NullPointerExceptions in tests).
Is it ok to refactor the createBlamer() method so that the JenkinsFacade gets passed as an argument and can be mocked? Otherwise there is no possibility of unit testing this class.
Actually I think I can do it without an extra argument. I just noticed the @VisibleForTesting annotation and will use that to prevent public API changes, but I still need to change the original class a bit.
Yes, this actually is required (and that is the concept around the facade if you have a look at some other usages). When you refactor the method just create an additional overloaded method (package private access with @VisisbleForTesting annotation) so the current client code does not need to change.