What's wrong with: Tá gruaig dubh ar an fhreastalaí seo
The other is usually interpreted more as 'This waiter has black hair'. Slight differencr.
So this sentence is more literally "The hair of this waiter is black", and gruaig an fhreastalaí is using the tuiseal ginideach?
Or as it's expressed. The genitive can be 'x of y' or 'x'S y'. Both are genitive.