Childish thing to say to a black sheep (3,3)

'childish thing to say' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'a black sheep' is the wordplay.
I cannot really understand how this works, but
'a' is present in the answer.
'black' could be 'b' (as in pencil classifications) and 'b' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

'to' is the link.

Can you help me to learn more?