Black rainbows or Moonbows coincide with a full moon and are the result of light refracting through water droplets in the atmosphere.
Moonbows are relatively faint, due to the smaller amount of light reflected from the surface of the moon. They are always in the opposite part of the sky from the moon. Because the light is usually too faint to excite the cone color receptors in human eyes, it is difficult for the human eye to discern colors in a moonbow.
For true moonbows the moon must be low in the sky (less than 42 degrees and preferably lower) and the sky must be very dark. Since the sky is still light on a rising full moon, this means they can only be observed 2 to 3 hours before sunrise, a time with few observers. And, of course, there must be rain falling opposite the moon. This combination of requirements makes moonbows much rarer than rainbows produced by the sun. Moonbows may also be visible when rain falls during full moon rise at extreme latitudes during the winter months, when the prevalence of the hours of darkness give more opportunity for the phenomenon to be observed.