Blue Lake is a city in Humboldt County, California, United States. Blue Lake is located on the Mad River in a deep valley, 16 miles northeas…Blue Lake is a city in Humboldt County, California, United States. Blue Lake is located on the Mad River in a deep valley, 16 miles northeast of Eureka, at an elevation of 131 feet. Its population is 1,208 as of the 2020 census, down from 1,253 from the 2010 census.