The pupil of a rabbit's eye is black, the blue part is the white of the eye, in low light the rabbit will dilate the pupil and it looks like a black eye, you can try to put it in a place with strong light, such as letting the light to see if it's still black, that means your rabbit originally has black eyes, the color of the rabbit's eyes will not change with the age of the rabbit's had long. I hope my answer is helpful to you and wish your rabbit a healthy and lively growth.