It means a union of opposites, like the black and white steeds.

They set off on a black steed in torrential rain.

The rider of that charging black steed was a young woman, but like none I had ever seen before.

The black steed stared at the place where his adversary, his other half, had last stood.

Mounted upon their sturdy' black steeds, they swept down into the fray.

If anything, the black steeds of the Bloodriders seemed immune to fatigue, pain, and fear.

He was a tall, gallant cavalier, mounted on a black steed.

"Shall I be clothed in armor and ride upon a great black steed ?"

The rat changed into a gigantic black steed, pawing air and snorting fire.

A woman on a black steed trailed them, scrutinizing their posture or the horses', or both.