It's an older version of the nickname 'Black Country'.
The Black Country today lies within the borders of Dudley, Sandwell and Walsall Boroughs and Wolverhampton City but does not actually coincide with any "official" boundaries. There is no definitive map of the Black Country. In fact, it almost defies definition. However, we all know where we are from, even if we might have slightly differing views over exactly what is in and what's not in our dark region. Some places are like that!
The Black Country was not always so-called, but the industrial revolution transformed the area now known by this name. Indeed, our black country was home to that revolution. By the mid-nineteenth century it had been described as 'The Iron Country', the 'Staffordshire Mining District' and finally the Black Country. In 1868 Elihu Burritt, who trained as a blacksmith before he became the American Consul in Birmingham, published his book 'Walks in the Black Country and its Green Borderland'.
The name Black Country derives from the important coal mining industry, the spoil from which was deposited around the pits, turning green to black. And it owes something to the overwhelming presence of smoke from the iron works that made our homeland "black by day, red by night".
Dickens visited the Black Country of the Midlands for the first time in 1838, and the wretched life of the people he witnessed there made a deep impression upon him. He wrote to his wife, "[w]e were compelled to come [to Shrewsbury] by way of Birmingham and Wolverhampton ... through miles of cinder-paths and blazing furnaces and roaring steam engines, and such a mass of dirt gloom and misery as I never before witnessed".