"Yuck! This cheese is black!"
Translation:Hyi! Tämä juusto on mustaa!
I actually wouldn't necessarily say it's wrong to say "musta" either! Cheese can be a mass noun as well as a countable noun. With "musta" it sounds like the whole piece of cheese is black, with "mustaa" there's black in it but not necessarily the whole cheese. So I'd say that this English sentence doesn't really tell which one is the case with this particular cheese.