The name is not English, it is Greek, very ancient. It consists of 'mel', meaning 'dark', and 'antha', meaning 'flower': thus Melantha means 'Dark Flower'. This has two references: either to a girl with dark hair and eyes, OR to a girl with a dark side to her personality.
"Melantha" might be a variant of the Greek name "Melantho" (accent on the "a") which appears in the epic poem "The Odyssey," and its masculine form, "Melanthios," which also appears here.