It's always worth checking in a dictionary to see whether a phrase is a common idiom. In this case, you've come across a very common expression which you could have found in any dictionary. Here's what the Cambridge Learners' Dictionary says:

The black sheep of the family:

a <a href="https://dictionary.cambridge.org/dictionary/english/person" target="_blank">person</a> who has done something <a href="https://dictionary.cambridge.org/dictionary/english/bad" target="_blank">bad</a> that <a href="https://dictionary.cambridge.org/dictionary/english/bring" target="_blank">brings</a> <a href="https://dictionary.cambridge.org/dictionary/english/embarrassment" target="_blank">embarrassment</a> or <a href="https://dictionary.cambridge.org/dictionary/english/shame" target="_blank">shame</a> to his or her <a href="https://dictionary.cambridge.org/dictionary/english/family" target="_blank">family</a>:

google "what does black sheep mean?" gets lots of reasonable explanations.

Usually black sheep (when used to describe a person) means outsider or one who doesn't fit in. As mentioned in the definitions by Chris and Su Ki, and Danilo, there can be an element of disfavor or shame involved.

In the song, black sheep seems to mean the one who didn't fit in because he was a wild child and likely gave his parents trouble. It's a good song and it's interesting to see that people are still listening to it. I know it best from 1974 when Terry Jacks made it famous.

Chris ah ! Yes as you said I might find wrong dictionary!

The Oxford Learner's Dictionaries are an excellent resource.

Definition of black sheep noun from the Oxford Advanced Learner's Dictionary

black sheep <em style="color: rgb(255, 255, 255);">noun</em>

 

<em>the black sheep of the family</em>