Other stains in wood are also defined in the glossary included below. Blue stain is caused by microscopic fungi that commonly infect only the sapwood of trees, using parts of the sapwood (including simple sugars and starches) for food. They cannot grow in heartwood or most wetwood that does not contain the necessary food substances. Blue stain fungi are prone to cause bluish or grayish discoloration of the wood but they do not cause decay. Blue stain has no effect on the strength of the wood. In a cross section of wood containing blue stain, the discoloration often appears as pie-shaped wedges, oriented radially, corresponding to the direction of the wood rays. The discoloration may completely cover the sapwood or may appear as specks, spots, streaks, or patches of various color intensities. Blue stains come in various colors, with the most common ranging from blue to bluish black and gray to brown, although various shades of yellow, orange, purple, and red sometimes appear. The exact color of the stain depends on the infecting organism and the species and moisture content of the wood. Sometimes other types of stains are confused with blue stain, such as chemical brown stain. Glossary Decay Fungi. Two major kinds of decay fungi are recognized: brown rot and white rot. Brown-rot fungi extensively remove only cellulose from the wood, which takes on a browner color and can crack across the grain, shrink, collapse, and be crushed into powder. White-rot fungi remove both lignin and cellulose from the wood, which may lose color and appear “whiter” than normal. The wood does not crack across the grain, and until severely degraded, it retains its outward dimensions, does not shrink or collapse, and often feels light and spongy. Brown-rot fungi commonly colonize softwoods (conifers), and white-rot fungi commonly occur in hardwoods (broad-leaved trees), but both brown- and white-rot fungi occasionally colonize both types of wood. Blue Stain. A bluish or grayish discoloration of sapwood caused by the growth of certain dark-colored fungi on the surface and in the interior of the wood; growth is made possible by the same conditions that favor the growth of other fungi. Chemical Brown Stain. A chemical discoloration of wood, which sometimes occurs during air or kiln drying of several species, apparently caused by the concentration and modification of extractives. Iron Tannate. A bluish black surface stain on oak and other tannin-bearing woods following contact of the wet wood with iron or with water in which iron is dissolved. Mineral Stain. An olive to greenish black or brown discoloration in hardwoods, particularly maple, caused by bird peck or other injury and found either in mass discoloration or mineral streaks. The mineral associated with such streaks is frequently calcium oxalate, which has a tendency to dull machining knives. Stain. A discoloration in wood that may be caused by micro-organisms, metals, or chemicals. The term also applies to materials used to impart color to wood. Sticker Stain. A gray to blue or brown chemical stain occurring on and beneath the surface of boards where they are in contact with stickers (also fungal sapstain when found only in the sticker area). Water Stain. A yellowish to blackish surface discoloration caused by water that dripped onto the wood during drying. Weathering. A very thin grayish brown surface discoloration on lumber that has been exposed to the weather for a long time. 
The following is excerpted from an excellent paper on the subject from the US Department of Agriculture Forest Service, Forest Products Laboratory in Madison, Wisconsin. As noted here, spalted wood weakens and decays as the spalting process progresses, while wood with blue stain discolors, but does not decay. 
