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.

L. indigo grows on the ground in both deciduous and coniferous forests, where it forms mycorrhizal associations with a broad range of trees.

Originally described in 1822 as Agaricus indigo by American mycologist Lewis David de Schweinitz,[3] the species was later transferred to the genus Lactarius in 1838 by the Swede Elias Magnus Fries.

[4] German botanist Otto Kuntze called it Lactifluus indigo in his 1891 treatise Revisio Generum Plantarum,[2] but the suggested name change was not adopted by others.

Like many other mushrooms, L. 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.

Under appropriate environmental conditions of temperature, humidity, and nutrient availability, the visible reproductive structures (fruit bodies) are formed.

It is often zonate: marked with concentric lines that form alternating pale and darker zones, and the cap may have dark blue spots, especially towards the edge.

[15]The flesh is pallid to bluish in color, slowly turning greenish after being exposed to air; its taste is mild to slightly acrid.

[21] Hesler and Smith, who first described the variant based on specimens found in Brazoria County, Texas, described its typical habitat as "along [the] sides of a muddy ditch under grasses and weeds, [with] loblolly pine nearby".

[8][9] Viewed with a light microscope, the spores are translucent (hyaline), elliptical to nearly spherical in shape, with amyloid warts, and have dimensions of 7–9 by 5.5–7.5 μm.

[12] The hymenium is the spore-producing tissue layer of the fruit body, and consists of hyphae that extend into the gills and terminate as end cells.

[26] The blue color of L. indigo is due to (7-isopropenyl-4-methylazulen-1-yl)methyl stearate, an organic derivative of azulene, which is biosynthesised from a sesquiterpene very similar to matricin, the precursor for chamazulene.

Other Lactarius species with some blue color include the "silver-blue milky" (L. paradoxus), found in eastern North America,[21] which has a grayish-blue cap when young, but it has reddish-brown to purple-brown latex and gills.

[28] Lactarius indigo is distributed throughout southern and eastern North America but is most common along the Gulf Coast, Mexico, and Guatemala.

[38] Reflecting their close relationships with trees, the fruit bodies of L. indigo are typically found growing on the ground, scattered or in groups, in both deciduous and coniferous forests.

For example, American mycologist David Arora considers it a "superior edible",[9] while a field guide on Kansas fungi rates it as "mediocre in quality".

[46] In Mexico, individuals harvest the wild mushrooms for sale at farmers' markets, typically from June to November;[12] they are considered a "second class" species for consumption.