When we see something as black, it signifies the absence of light in that particular object. The composition of these rain-bearing clouds is rather thick because these clouds contain lots of thick fog, almost ready to condense into water droplets. These kinds of clouds do not allow much light to pass through. If the scattered light of the sky does not get passage, the obstructing object looks black. In this case, the large obstruction.
Why do rain clouds appear black ?