Blyth is a port and seaside town as well as a civil parish in southeast Northumberland, England. It lies on the coast, to the south of the Riv…Blyth is a port and seaside town as well as a civil parish in southeast Northumberland, England. It lies on the coast, to the south of the River Blyth. It has a population of 39,731 as of the 2021 census, up 6% from the 2011 census and population of 37,347.