Translation:– Do you want some cream? – Black, please.
Yes, that's how I'd translate it into English as well. To your question about why is it in essive... I wish I knew :D No but, that's just how Finnish works. I have to read about essive myself as well and it's a case for expressing being a certain way. Basically you can also imagine that as saying "coffee in black", even though no one would really use that expression ever in English.
But really what you yourself just said is what it means, I just don't know how easy the case is to comprehend through English sentences as sometimes the Finnish cases are almost impossible to compare to English grammar.
WARNING: NON NATIVE FINNISH SPEAKER... It may be unrelated, but i understand that sometimes this case is used as a 'temporary state of being' so for example 'as a child' = Lapsena, because a person is not a child forever(?).
Perhaps the individual in this example has decided to take their coffee black, but of course can also add milk/cream/sugar later if it is too bitter. :D
Of course there could be a more logical explanation!
Yes, it's bit hard to explain because the essive case is quite unique but it means the state of being something and can be used also of what something used to be, like 'lapsena' = 'as a child'. The grammatical tense will be the one to show if it's about the past, after all.
To the coffee one: usually people here who order their coffee black, will always drink it black only and don't even intend to add any cream or milk to it :D I could imagine you'd express it (to the cashier) if you wanted to test drinking it black first.