Their are multiple reasons plants aren't black colored. Black represents the absorption of all wavelengths of visible light. In other words, none of the light is reflected. As a result, black colored objects tend to get hot in the sunlight since they absorb so much radiation. If plants absorbed too much radiation, their delicate tissues would not be able to handle the heat stress and the plants would wilt and suffer. In addition, the green coloration that dominates the plant world is the result of the pigment chlorophyll. Chlorophyll absorbs a particular band of radiation because it requires that specific energy level to initiate photosynthesis. Finally, plants tend to have brightly colored flowers to attract insects for pollination. If they were black colored, they would not stand out to nearby insects.
We see green when looking at a leaf because chlorophyll in the leaf absorbs vilet, blue and red light while reflecting green light. For plants to be black, they would have to reflect black light, and absorb all other light wavelengths. That is not possible, and would be much too streeful on the plant leaves.
Plants are are not black or dark in pigment because they cannot absorb that much light. Much light is reflected or simply passes through thin leaves or is intentionally dissipated by protective pigments. If plants absorbed all light they would appear black.