The chef was doing a cake with a wheel gum paste on the top. He noticed that the wheel is getting cracked on the bottom so he said:
"I grabbed a bag of black royal"
So "black royal" here is just a colour or it's a kind of chocolate? When I searched, it was about an institute and nothing about colours.
(Not a teacher)
I did a quick search and found that it is a type of icing (frosting in American English). I found this description on a recipe website:
Royal icing is the thick, solid icing that is often seen on wedding cakes and Christmas cakes. Made of egg whites and sugar royal icing is white but different colors can be made by using specliast food colorings.
Presumably, the chef here used black food colouring.