Translation:That board is not black, but green.
Yes, as in "table of figures", apparently.
You're right. And often they're green, even though we also call them blackboards.
("Board", "blackboard", and "chalkboard" are synonyms, though usage of any one of these may vary among regions.)
"This painting is not black, but green" should one of the answers accepted. I understand the context of this sentence where duolingo wants us to know 'tableau' can refer to a board, but this sentence makes perfect sense with tableau also meaning a painting. If duolingo wants to force us to translate tableau into board only, they should come up with a different sentence. We shouldn't be penalized for this.