It is indeed very unfortunate to have been hit in the eye. A blunt injury
such as a fist injury can cause a swelling in the back of the eye called as berlin's edema. This can be confirmed by a dilated eye exam by an ophthalmologist. If present it can be treated with the help of anti-inflammatory eye drops. If the retina
is found to be normal then this black spot ( which probably floats) could be explained by the presence of a PVD- posterior vitreous detachment
( probably due to the injury). It is better to visit an ophthalmologist at this stage to confirm the above.