Where is the black country? Is this as opposed to the white country?
That’s racist!
The area was very much the cradle of the industrial revolution, particularly heavy industry - manufacturing, metal bashing, engineering - later auto construction. Coventry was like a British Detroit - all the main British auto manufacturers - Rolls Royce, Austin, Triumph, BSA, Morris, MG, Leyland and so on were based there.
Anyway, in the very early days all that industry meant steam power generated by firing coal. Lots and lots of it. Result: the sky would literally turn black from all the smoke and pollution - hence "black country". It's not an exageration either - the lack of sunlight caused a lot of problems - rickets was notoriously prevalent amongst children.