Русско-английский словарь пословиц и поговорок
ЧЕРНОЕ ОТ СТИРКИ НЕ БЕЛЕЕТ
See Горбатого могила исправит (Г)
Cf:Bad is the wool that cannot be dyed (Am.). It is a bad cloth that will take no colo(u)r (Am.,Br.). It's ill wool that will take no dye (Br.)