The black spots are also called floaters. It is normal for them to become more prominent when looking at something bright, such as white paper, a blue sky or in snow field. So it is better to wear dark sunglasses when outside.
It is a common problem. In most cases, eye floaters do not attract serious attention because everyone experiences them from time to time and no ill effects will be caused. In particular, eye floaters are the exact result while looking directly at a light background. They occur within the eyeballâ€™s vitreous fluid and can not be removed simply by rubbing the front eye.