As the name suggests, this is a tattoo that heavily focuses on shading with black ink. This provides depth and an almost 3D effect without using any colour. Highlights are usually added when the tattoo is completed using white ink to make the tattoo really pop. White ink is also used to smooth out sharp transitions between different shades.
Black and grey tattoos in Thailand are usually the go to for realism & portrait work, but are also applicable to a number of other design styles like traditional.