 Lactarius indigo , commonly known as the indigo milk cap , the indigo ( or blue ) <unk> , or the blue milk mushroom , is a species of agaric fungus in the family <unk> . A widely distributed species , it grows naturally in eastern North America , East Asia , and Central America ; it has also been reported in southern France . L. indigo grows on the ground in both deciduous and coniferous forests , where it forms mycorrhizal associations with a broad range of trees . The fruit body color ranges from dark blue in fresh specimens to pale blue @-@ gray in older ones . The milk , or latex , that <unk> when the mushroom tissue is cut or broken — a feature common to all members of the Lactarius genus — is also indigo blue , but slowly turns green upon exposure to air . The cap has a diameter of 5 to 15 cm ( 2 to 6 in ) , and the stem is 2 to 8 cm ( 0 @.@ 8 to 3 in ) tall and 1 to 2 @.@ 5 cm ( 0 @.@ 4 to 1 @.@ 0 in ) thick . It is an edible mushroom , and is sold in rural markets in China , Guatemala , and Mexico . 
