"Black mold" or "toxic mold" are popular ways to refer to the Stachybotrys genus of the fungus family, the reason for this being twofold: (1) The species of that genus are almost always black in color and (2) a number of species emit fungal toxins or mycotoxins. The latter fact has impressed its stigma on all the other species due to the fact that distinguishing one type of Stachybotris fungus from another is impossible without the use of specialized equipment.
There are other genera of fungi exhibiting a black color. Still others produce poisonous spores or reproductive cells and emit them into the air. However, in most instances, what people refer to when they speak of "black mold" is the species chartarum. That is probably because this species is notorious for causing illnesses in animals and human beings. Other names for Stachybotrys charatrum are S. atra or S. alternans.
What they look like and where they thrive
Stachybotrys is among the smaller kinds of fungi. Its color is usually black, tinged with a hint of green. Its distribution is on a global scale which probably accounts for the fact that they have come to represent all types of poisonous fungi. The colonies occur as circular spots on walls and ceilings. They look for wet surfaces made of cellulose-rich materials which include wood and paper among others.
The likeliest places to find Stachybotris in the outdoors are dead trees. Tree stumps and logs are the ideal places to look for samples of fungi. Fungi are also plentiful in areas that are flood-prone.
The air is free from fungal spores during winter up to the early autumn. July is the month when the spores germinate and fungi proliferate.
In your house, you should be on the look-out for locations that are enclosed. The moisture in these areas is not readily released into the surrounding air. Such areas include closets, cabinets, drawers and enclosures in the wall or ceiling. Those are the places which mold-removal people typically examine for the presence of "toxic molds." Other areas where molds love to grow are inaccessible portions of the wall/floor which regularly come in contact with water. For instance, you should be able to find molds behind the kitchen or bathroom sink. The same is true for portions of the wall behind bathtubs or bath showers. The hosts that molds love to attach themselves to are rich in cellulose content. These materials include wood, paper and paper products. Your cotton, linen and rayon fabrics may also get moldy if stored for a long time in the closet.
The toxin comes from the spores
The manner in which Stachybotrys reproduces is by shooting out microscopic spores into the air. These are then wafted to places that are accessible to the wind carrying them. Spores are known to travel for tens of miles from their place of origin. If they manage to enter a house, they propagate colonies of "black mold." As a result, the air in that house becomes impregnated by these spores. People and animals living in that house are at a risk of developing the symptoms of mycotoxin poisoning.