That expression is correct, but that is an exception rather than common usage. I should also point out that dark chocolate is alternatively called black chocolate.
I can never tell when to use the different words with es ez like voulez, voules, vous, or vouz
Not sure, because it's not a question here. In english, do you want a black horse without any question mark would be wrong. THe other possibility could be "You do want a black horse", as a way of stressing the desire.
Oh, yes, thank you. I completely disregarded the missing question mark. Probably because stating: "you" want a black horse! seems a bit very strange.