Under appropriate environmental conditions of temperature, humidity, and nutrient availability, the visible reproductive structures ( fruit bodies) are formed. indigo develops from a nodule that forms within the underground mycelium, a mass of threadlike fungal cells called hyphae that make up the bulk of the organism. In central Mexico, it is known as añil, azul, hongo azul, zuin, and zuine it is also called quexque (meaning 'blue') in Veracruz and Puebla. Its names in the English vernacular include the 'indigo milk cap', the 'indigo Lactarius', the 'blue milk mushroom', and the 'blue Lactarius'.

The specific epithet indigo is derived from the Latin word meaning 'indigo blue'.

hemicyaneus may be considered as mileposts along the road to L. indigo. L. chelidonium and its variety chelidonioides, L. paradoxus, and L. The climax is reached in L. indigo which is blue throughout. The gradual development of blue to violet pigmentation as one progresses from species to species is an interesting phenomenon deserving further study. In 1979, they revised their opinions on the organization of subdivisions in the genus Lactarius, and instead placed L. indigo in subgenus Lactarius based on the color of latex and the subsequent color changes observed after exposure to air. Hesler and Smith, in their 1960 study of North American species of Lactarius, defined L. indigo as the type species of subsection Caerulei, a group characterized by blue latex and a sticky, blue cap. German botanist Otto Kuntze called it Lactifluus indigo in his 1891 treatise Revisio Generum Plantarum, but the suggested name change was not adopted by others. Originally described in 1822 as Agaricus indigo by American mycologist Lewis David de Schweinitz, the species was later transferred to the genus Lactarius in 1838 by the Swede Elias Magnus Fries. In Honduras, the mushroom is called a chora, and is generally eaten with egg generally as a side dish for a bigger meal. It is an edible mushroom, and is sold in rural markets in China, Guatemala, and Mexico. The milk, or latex, that oozes when the mushroom tissue is cut or broken - a feature common to all members of the genus Lactarius - is also indigo blue, but slowly turns green upon exposure to air. The fruit body color ranges from dark blue in fresh specimens to pale blue-gray in older ones. L. indigo grows on the ground in both deciduous and coniferous forests, where it forms mycorrhizal associations with a broad range of trees. It is a widely distributed species, growing naturally in eastern North America, East Asia, and Central America it has also been reported in southern France. Lactarius indigo, commonly known as the indigo milk cap, indigo milky, the indigo (or blue) lactarius, or the blue milk mushroom, is a species of agaric fungus in the family Russulaceae.

Indigo milk cap mushrooms recipe