There is a traditional folksong reference to "black sheep" as a term for slaves, appearing in a song where "blackbirds" has the same meaning. It's in the song "Congo River", collected by Stan Hugill, and appearing on Tom Lewis' web page with the song's lyrics . Here are those lyrics:

Say - was you ever on the Congo River? (Blow, boys, blow.)
Where fever makes the white man shiver. (Blow, m' bully boys, blow)

A 'blackbird' ship come down the river,
Her masts and spars they shone like silver.

Who do you think was the skipper of her?
It's old Bully Hayes, he's a sailor robber.

What do you think that they had for breakfast?
The starboard side of an old sou'wester.

What do you think that they had for dinner?
Hot water broth but a little bit thinner.

What do you think that they had for supper?
Belaying-pin pie and a roll in the scupper.

What do you think that they had for cargo?
"Black Sheep" that had run the embargo.

Blow today and blow tomorrow,
Blow me from this ship of sorrow.

Blow today and blow forever,
Blow me down that Congo River.

So, it appears that the Liverpool Philharmonic was correct about one use of the term, but not about its use in the "Baa baa black sheep" context.