Part of an incised stoneware blacking bottle, marked with the manufacturer’s name and address and ‘BLACKI[NG BOTTLE]’. Blacking bottles were identified in this manner to avoid the excise duty levied on stoneware bottles in the period 1817-1834, from which they were exempt. Found on the Thames foreshore.
See now the comments below. The ‘BLACK’ is part of the word BLACKFRIARS. The fragment comes not from a Blacking Bottle but from an early stoneware Hamilton.