Lactarius indigo

Common Name: None.
Description: Cap 5-15 cm across, convex-depressed with an inrolled margin at first; indigo blue when fresh, fading to grayish, then having a silvery luster, with deep green areas where bruised; sticky, smooth, zoned.
Habitat: scattered or in groups on soil in oak and pine woods. Common in the Southeast, rarer farther north.
Edible: Yes.

Source: Mushrooms of North America by Roger Phillips.