In fact the rise in single-parent family has cross cut all racial and ethnic lines. However, the rate of growth is higher among black families then among other groups. Note that regardless of family type, blacks, as well as Hispanic families, have lower median income. The argument that single parenthood is the cause of poverty assumes that the single parent is the cause and the effect is poverty. This is not the case, but rather the opposite is true.
Poverty causes single parenthood.
Poverty prevents and disrupts marriage. In the U.S. we assume that the male is the primary means of economic support for a family. This has several consequences. Poverty is even more strongly associated with births outside marriage. Poor inner-city women experience a severe shortage of men who would make suitable marriage partners because many of the men available to them are either unemployed, imprisoned, or killed.