If a seller is describing the hens as Black Rock, they should be the cross you mention. That is the accepted norm for this hybrid. They come in at around 2kg in weight, so an average size for a hybrid.
If you are picking the bird up, ask if you can have look at the rearing pens. You should expect things to be a bit more crowded that you would ideally have at home, but everyone should be active and alert. It might not be spotless, but any sign of filth or neglect, then walk away.
If a hen comes from a good breeder, she should be sound and hardy