Somewhere up above someone suggested that rain clouds are black because they block the light. For some cases - when the light is behind the cloud - this is "an answer" but not an explanation.

A spherical blob, such as a raindrop, that's even slightly transparent will pass some light from behind, and when the light comes from the front will "turn around" part of the light and reflect it back. Neither effect is 100 per cent, so if there's "a sufficient density of blobs" the object (the cloud) looks darker when lighted from behind, and may be "somewhat grey" when the light is from the front.

When the light is on the front, the cloud looks really black only when the individual drops get too big to remain spherical, and the light is just "scattered" within the cloud instead of being reflected straight back.

Small water drops that can remain suspended by "normal" air motions are very nearly spherical due to surface tension. When the drops get big enough to remain "up there" only in the presence of strong rising air currents, the air blast "warps them" to irregular shapes that no longer are "good reflectors." The light is no longer reflected straight back to where it came from, but instead goes off into "somewhere else" in the cloud. Multiple - less than 100% - reflections results in all the light being absorbed in the cloud.

If a cloud starts to look black when the light is on the front side of the cloud, it means that the individual drops are big enough to require strong up-flowing air to remain in the cloud, which is a condition conducive to their eventual "falling down" as rain.

John