"The dress is blue and black!"
Translation:Kjolen er blå og svart!
"En kjole" is a masculine noun, so it takes the masculine version of any adjectives describing it as well.
"Blått" and "svart" are both masculine forms of their corresponding adjectives, while "blått" is the neuter form, and "svarte" is the definite/plural form. "Blått" is also the name of the colour itself.