All pigs, whatever their hair colour, have white skin. It is a common misconception that black haired pigs will have black skin. Meat from a Saddleback will look no different than meat from a Large White. It will not look bruised or off.
This is not the reason the abattoir wants to skin them though. Some abattoirs don't like coloured pigs because they don't want black hairs left in the dehairing bath that might end up on their white pigs.
Is there an alternative abattoir?