2003 Photo of Hartlepool - Seaton Carew. The black is sea coal that gets dredged up by the tides and dumped on the beach. Then it gets collected by local sea coalers who get up early in the morning to collect it with shovels. They then sell it or possibly use it themselves. In the past it was used to stoke fires.

Some history here. With old photos and a song from the Kirkleatham museumin Redcar.

The Young’uns have a Sea Coal song.

Referenced on page 89 of “The Parochial History and Antiquities of Stockton-upon-Tees: Including an account of the trade of the town, the navigation of the river, and of such parts in the neighborhood as have been connected with that place” Second Edition by John Brewster, M.A., Rector of Egglescliffe, Fellow of the Society of Antiquaries, Newcastle-On-Tyne.