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

'childish thing to say' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'a black sheep' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
'black' could be 'b' (abbreviation as in pencil hardness) and 'b' is found within the answer.
This explanation may well be incorrect...

'to' acts as a link.

Can you help me to learn more?