A google search will turn up lots of images and web pages on the topic "Staffordshire Blue Brick". Although not really appearing blue on closeup, at a distance the effect is more clear. The clay must have enough iron to both stain it and act as a flux in the reduction kiln atmosphere. The degree of atmospheric consistency inside the kiln will determine the range of colors produced. A potter can achieve this effect by firing a red earthenware in reduction (e.g. to cone 2R) or firing a middle temperature red burning body to cone 6R. Additions of iron oxide will enhance the effect, however thorough testing is needed to achieve the difficult balance of enough iron to get the color but not so much it will over-vitrify and bloat or melt.
A method of firing stoneware where the kiln air intakes and burners are set to restrict or eliminate oxygen in the kiln such that metallic oxides convert to their reduced metallic state.