Like all other categories of Chinese teas, Hei Cha is family of Chinese teas that are manufactured and processed with the intention to post-ferment. They are therefore an alternative name of 'aged teas'. Generally speaking the more they are aged, the better their quality is.
Hei Cha have been compressed largely in the post, with the purpose to facilitate their post-fermentation, storage and transportation. The modern varieties however range from loose, small nuggets/cake right up to big cakes and bricks.