Definition of "blackened"

[...] after journeying across pleasant agricultural countryside to Leicester and thence though the blackened and busy industrial area that stretches almost unbrokenly for over 100 miles from Trent through Rotherham and Leeds to Keighley, [...].

1939 July, John D. Hewitt, “Some Notable British Main Lines: 1. Settle and Carlisle, L.M.S.R.”, in Railway Magazine, page 34