This is a problem that I have noticed every time "maïs" is said or written: French say "LE maïs" not "LES".
If it were "les maïs" you would have the verb and the adjective in agreement, ie in plural "les maïs sont noirs"
We don't refer to corn as a plural in english either. We don't say 'the corn are ready', we say 'the corn IS ready'. Nor do we refer to other words as plurals. We don't say 'i'm getting my hairs cut'. We use that as a singular as well.