"I was the black sheep of the family."
Translation:Byl jsem černá ovce rodiny.
I am not sure which indefinite article you mean, there is none around. If you mean the definite the in front of "black sheep" than that is dictated by the rules of the English grammar. There are no articles in Czech so those rules cannot apply.
ta černá ovce rodiny is that black sheep of the family, it is some specific black sheep of the family we were already speaking about.