i thought nere was black - does it change gender?
Yup. Adjectives do.
my answer was: it isn't a black shoe. why is it wrong?
Nothing wrong with your answer, you could report it so they will consider that option too.
I wrote "the shoe is not black". Not sure why that is wrong.
there is a slight difference, maybe 'this is not a black shoe' would mean 'black shoe' is a thing