A quote from the British zoologist, Renshaw (1904)

An alleged change of colour in the skin of the Blaauwbok after death has given rise to some comment. Pennant, in his ‘History of Quadrupeds', says;- “Colour, when alive, a fine blue of a velvet appearance; when dead changes to a bluish grey with a mixture of white.’ Dr. Sparrman, who travelled in South Africa during 1772-1776, in mentioning the Blaauwbok, observes: “On this subject the reader may likewise turn to Mr. Pennant’s Blue Antelope” and also says: “The colour of this creature when alive is said to resemble that of blue velvet, but when it is dead it is of a leaden colour.” Le Vaillant, who obtained a Blaauwbok bull in December, 1781, states that the colour of the animal was faint blue inclining to grey, with snow-white belly, the head being above all beautifully spotted with white; but, he adds (Travels in Africa,’ vol i. p. 132). “I did not observe, as Dr. Sparrman says, that this Antelope when alive resembles blue velvet, and that when dead the skin changes its colour; living or dead it appeared to me always alike. The tints of that which I brought with me never varied.”
At first sight it would thus seem that the statement of Le Vaillant contradicts that of Sparrman, and also indirectly that of Pennant, but we must remember in some Antelopes, such as Eland and Kudu, the hair becomes so scanty that the bluish hide shows beneath it in old age; and this hide, after post-mortem drying, becomes black or “leaden colour.” Further, this change due to drying is actually recorded by Sir Cornwallis Harris as taking place in the Roan Antelope, the nearest living ally of the Blaauwbok, and we may therefore well assume that Le Vaillant expected to see some conspicuous change in the hairy covering itself due to chemical or other causes, such as has been observed to take place in the lilac breast-feathers of the newly-dead Gouldian Finch (Poephila gouldiae). If the first two or three Blaauwbok obtained were infirm old bulls, easily dispatched by the uncertain and primitive weapons of the old days, we can reasonably infer that the hide, denuded through age of most of the original hairy covering, would appear conspicuously bluish during life, and conspicuously black after post-mortem drying, and thus originate the colour-change legend.

… The blue-grey colour need not have been disadvantageous to it, for travellers have assured us that the boldly coloured Roan and Sable Antelopes, in spite of their great size, are often quite invisible in the broken lights and shadows of thick bush; and especially at night the neutral grayish tint was well adapted to protect the Blaauwbok, just as our own warships painted grey become practically invisible in the gloom of night.

Renshaw, G. Natural History Essays, ‘The Blaauwbok’ 1904:443-444