[N]くろさ　blackness

some い-Adjectives and な-adjectives can become nouns by replacing い into さ.

例　やさしい　kind➡やさしさ　kindness

　　おもしろい　interesting/funny➡おもしろさ　interestingness/funniness

　　便利べんりな　convenient➡便利べんりさ　convenience

※It isn’t suitable for all adjectives.