Not that the Black Road exists any more, of course.
"It was a long, diagonal cut across the black place if we wanted to keep to the road. Also, part of the terrain was hidden from our sight by a series of low, stone, hills. There were heavy grasses at the edge of the black and patches of it, here and there, about the foot of the hills. Bits of mist scudded among them, and faint, vapourous clouds hovered in all the hollows. The sky, seen through the atmosphere that hung about the place, was several shades darker, with a smeared, sooty tone to it. A silence that was not the same as stillness lay upon it, almost as though some unseen entity were poised, holding its breath...
"The black grasses, over which I had leaped, were twined about his ankles and legs. Even as he hacked at them, others lashed about as though seeking to capture his sword arm...
"We entered the black area, and it was like riding into a World War II newsreel. Remote though near at hand, stark, depressing, grim. Even the creaking and the hoof falls were somehow muffled, made to seem more distant. A faint, persistent ringing began in my ears. The grasses beside the road stirred as we passed, though I kept well away from them. We passed through several patches of mist. They were odourless, but our breathing grew laboured on each occasion."
'The Guns of Avalon', Chapter 8
It appears that most of the Chaosite army which came along the Black Road required it to avoiding becoming sick in the highly-ordered shadows close to Amber, and Amber itself. Only when fighting did they leave the road, an even then only for limited periods.