From the renowned artist Bob Ross, host of the famous TV show "The Joy of Painting".
Liquid Black is one of three Bob Ross Liquid Base coats essential to the Bob Ross Wet-on-Wet Technique. The method involves the use of these special slow drying mediums to allow blending and mixing paint on the canvas.
They can also be used as thinners for other colours.
The idea that a thin paint will stick to a thick paint is the basis for this entire technique.