Pure black is the most difficult colour to achieve. Black fabrics that you see in the store are often simply very dark blues, purples and greens. The best way to get black is to dye the fabric multiple times. When you take a black fabric from a fabric store and remove the colour with bleach or thiourea dioxide (a chemical used for removing dye, it is not nearly as harmful to fabric as bleach is), you often end up with a blue, green or orange fabric, because it was been dyed that colour first to make the black over dye richer and darker.
Logwood, cutch, and iron are natural dyes that will give a deep navy, black or brown. Iron is a mordant that adds a very dark saddened green brown colour to fabric, therefore when it is used as a mordant with a dark colourant it will give a warm black. Logwood is derived from a tree and will give rich blue to purple blacks, therefore a cool black.
Black dyes derived from logwood were popular in the 19th century when modesty through wearing black clothing was stylish. Men wore black smoking jackets in the evening and women wore shimmery black silk gowns. In contrast brown as a dye was humble and not often desired by the upper classes. Peasants and individuals who did not dwell on vanity, such as monks, would wear brown cloaks.
Black and brown to me mean very different things. Black gives contrast and definition, a single black line can say so much, and a dark black horizon line can define space, as can a darkened shadow. Black can be used to portray darkness and mystery, I find the black that I see in everyday life gives clarity and shows dimension. To me brown is rich like the earth, it provides nourishment. A rich brown compost has promise, and new growth will always spring up from it.