"– Do you want some cream? – Black, please."
Translation:– Haluatko kermaa? – Mustana, kiitos.
Musta kiitos is black please, the question is about cream not coffee,
No, it's about the coffee. There is no black cream, at least not that I would know of... Mustana is an essive case which means 'as (a) something' or 'in a certain way', it's kinda hard to explain in English. But imagine it as saying "coffee in black". So you are meaning that you want to drink your coffee as black, not with cream.
If it was about the cream, then you would say "Mustaa (kermaa), kiitos."
If it was about the cream, the whole thing should be "Haluatko jotain kermaa? Mustaa, kiitos." Jotain implies "one of these creams" (jotain näistä kermoista) and the answer would be "The black one, please."
I agree that I've never seen black cream in my life. Best was lakritsi or salmiakki flavoured cream, which was still grey.