Thank you very much for your explanation.
The problem arose because I was told that "black sheep" and "white crow" are not the same things. Historically. of course. not. But now I have a feeling that the expression "Black sheep" has lost its historically negative connotation and it is used in the meaning that Nata described. What about this "white crow"? I failed to find this idiom in the dictionary. Maybe, it doesn't exist? and the native speakers do not use it?