Nouns ending in -eau are usually masculine.
Is tableau masculine or feminine?
letableau
The rule behind it
93.1% of -eau nouns are masculine
In context
C'est le tableau qu'il a peint.
That is the picture that he painted.
Un tableau ne doit pas être obligatoirement noir.
A blackboard doesn't have to be black.
Comparez votre phrase avec celle sur le tableau.
Compare your sentence with the one on the blackboard.