what is mean "Black Bess"?

Black Bess was a horse, allegedly owned and ridden by 'Dick Turpin', a highwayman.
If Setanta chimes in - He will utterly expand on this.
I will enjoy his post, should he choose to do so.

Depends on the context. It's a horse in a famous old book. It is also a limited edition Bucatti.

It's a famous horse named as Black Bess