It is easy to translate "black sheep" literally: ovis nigra. I suspect that this phrase does not have the same meaning as in English (and Finnish), judging by its absence in literature — assuming my searches were correct. Is there a Latin phrase that means a disreputable member in a group, preferably classical?
superbis iuvenibus the 'proud young men' (dative pl) are described as
siccis herbis 'withered crops.' (1310 - 20 de Lisle Psalter:)
However, this is closer to "He's such a disappointment," than "The black sheep of the family." Alternatively,
impius, (3) undutiful (Ainsworth & Mead)
Filius impius in patrem (Tacitus)