Here's another article which gives more information about this kind of thangka painting. It is interesting that Jeff Watt calls them Black ground paintings, i would have thought they would be called black background paintings but then he's the art scholar, not me LOL!
http://www.tricycle.com/blog/himalayan-buddhist-art-101-black-ground-paintingsHimalayan Buddhist Art 101: Black Ground PaintingsPosted by Alex Caring-Lobel on 07 Feb 2013
Buddhist practice and Buddhist art have been inseparable in the Himalayas ever since Buddhism arrived to the region in the eighth century. But for the casual observer it can be difficult to make sense of the complex iconography. Not to worry—Himalayan art scholar Jeff Watt is here to help. In this "Himalayan Buddhist Art 101" series, Jeff is making sense of this rich artistic tradition by presenting weekly images from the Himalayan Art Resources archives and explaining their roles in the Buddhist tradition. Himalayan Buddhist Art 101: Black Ground Paintings
Black ground paintings are compositions in which the ground preparation for the canvas is of a black color or the background of the composition is painted black. This type of painting, with its unique and recognizable appearance, has an early historical source and ritual component.
Mahakala - Chaturbhuja (Four-hands). Tibet, 1700–1799. Drukpa (Kagyu)
and Buddhist lineages. 86.36x67.95cm (34x26.75in). Ground mineral pigment, fine gold line,
black background on cotton. Collection of Rubin Museum of Art. Traditionally, the iconographic subject for these paintings is limited to wrathful deities. These deities can be either protectors, such as Mahakala and Shri Devi, or meditational models, such as Vajrabhairava and Hevajra, as long as they are wrathful or semi-wrathful in appearance.
The surge in popularity of black ground paintings can be dated to the 14th and 15th centuries. The origins of the style date far earlier, rooted in the various tantric texts of the 9th to 12th centuries. The two most important examples of these texts are the twenty-five chapter and fifty-chapter versions of the Mahakala Tantra.
Shri Devi - Magzor Gyalmo. Tibet, 1800–1899. Buddhist lineage.
Ground mineral pigment on cotton. Private collection. Both texts contain sections where the devout practitioner is instructed to compose a painting for ritual use of his or her favored form of Mahakala using charnel ground cloth remnants and funerary ashes from a cemetery—hence the black coloring. Such instructions for creating drawn and painted likenesses of meditational deities and protectors are common to many tantric texts. Once a painting is created, it is kept secret and never displayed publicly, to be used only by that practitioner.
The Buddhist tantric system enumerates four tantric activities that correspond to four different colors: white for peaceful activities, yellow for activities of increase such as wealth and health, red for activities requirng subjugation or speed in their accomplishment, and black for wrathful activities, as exemplified by black ground paintings.