A quote from an encyclopaedia entry by Professor Skinner (2005)

From the evidence that can be gathered from the few specimens of the blue antelope that still exist and the writings and drawings of early travellers, we can build up a picture of their appearance in life. They appear to have been smaller than either of their near relatives the roan, H.equinus, or the sable, H.niger, with a height at the shoulder of about 1.0 m to 1.2 m, as compared with the oran at 1.6 m and sable at 1.3 m. Le Vaillant (1790) who shot and preserved the skin of a blue antelope described the colour as faint blue inclining to grey. They had dull whitish under parts which did not specially contrast with the colour of the flanks. The forehead and top of the muzzle were brown and this colour faded into the lighter colour of the sides of the face. They had a distinct white or at least a lighter coloured patch in front of the eyes and a light coloured upper lip. Their faces lacked the very distinct and extensive pattern of black and white seen in the sable H.niger or the somewhat similar pattern seen in the roan H.equinus. They had long and rather narrow pointed ears but not as long as in the roan and these lack the tuft of black hair at the tips seen in the roan. Their horns swept back in an even curve from the top of their heads but were much lighter in build than those of the roan or sable, and were flattened slightly on their inner sides as compared with those of a sable, which tend to be flatter on the outer sides, and the roan, which are generally rounder. From the existing material, the horns reached a length over the front curve of up to 0.61 m and had a series of 20 to 35 ridges. The tail, which just reached the hocks, was tufted. The tuft was darker than the general colour of the body.

Smithers RHN and Skinner J (2005). The mammals of the Southern African subregions. Cambridge University Press, Cambridge.