Rain-bearing clouds look black because

Rain-bearing clouds, such as nimbus or cumulonimbus, appear black or dark due to their high optical thickness and density. These clouds contain a massive concentration of minute water droplets and ice crystals formed by adiabatic cooling [2]. While typical clouds appear white because they scatter all wavelengths of sunlight equally (Mie scattering), rain-bearing clouds are significantly thicker and more expansive [2]. As the number and size of water droplets increase, the cloud becomes increasingly opaque. These droplets absorb and scatter the incoming solar radiation so effectively that very little light can penetrate through the base of the cloud [3]. Consequently, an observer on the ground sees a lack of brightness, making the cloud appear dark or black. This phenomenon is primarily a result of the high density of water particles absorbing and blocking the sunlight from reaching the lower layers.

Sources

1. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 21: Horizontal Distribution of Temperature > Transparency of Atmosphere > p. 283
2. [3] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 10: Water in the Atmosphere > Clouds > p. 87