there used to be a Ross Poultry factory down the road from where I used to live in Hampshire.
a friend of mine worked there and said they killed all the black chicks as they were male - so must have been a sex-linked broiler.
Almost correct

They wouldn't kill all the males.
All the chicks should be yellow, if a black chick appears it means one of the parents was a "sexing error"
In commercial broilers, the parents of the broiler bird come from two different strains, males from one, females from another.
So if a male gets through the day old sexing process along with the girls (his sisters) then it essentially is breeding with its own strain and then the black chicks will appear.
These male sexing errors are fairly easy to spot within the breeding flock, the females that escape through with the male strain are a lot harder to find!