In 1895 there was a brick maker who extracted the clay and the company was called Jones and Maxwell and 1911 Jones took over the site known as Pelaw brickworks. I studied the bricks when I did some research on a Hebburn Church.The clay is called Etruria Marl which is the best clay so I gather that could well be what is going on. You may well find blue/black bricks made by Jones around Hebburn but they are so hard you can't drill them. They are fired at 2000 degrees.