it should either be Black butterflies=kupu-kupu hitam or Black butterfly= Kupu hitam
No, some animals just have this 'double-names' in the singular form.
butterfly = kupu-kupu. butterflies = kupu-kupu.
dolphin = lumba-lumba. dolphins = lumba-lumba.
spider = laba-laba. spiders = laba-laba.
turtle = kura-kura. turtles = kura-kura.