Pennant’s description was based on a skin bought at Amsterdam and reported as brought from the Cape of Good Hope. His statement about the living animal’s colour was reported information.
Original text
A. with sharp-pointed, taper, arcuated horns bending backwards, marked with twenty prominent rings, but smooth towards their points, twenty inches long: ears sharp-pointed, above nine inches in length: larger than a buck: color, when alive, a fine blue; when dead, changes to a bluish grey, with a mixture of white: hairs long: beneath each eye a large white mark: belly white: tail seven inches long; the hairs at the end six inches. From a skin bought at Amsterdam, brought from the Cape of Good Hope; where they are found, but far up the country. This is the species which, from the length of its hair and form of the horns, connects this genus with that of the Goat.
Thomas Pennant, Synopsis of Quadrupeds (Chester: J. Monk, 1771), entry 13, “Blue,” pp. 24–25.