Blindness after any surgery is a possibility. There is only one artery that supplies the eye and with the patient face down, the eye is suspended. If there is pressure on the eye, trauma to this region or some very rare conditions where the artery is fragile and weak, loss of vision can occur.
I have seen this once in thirty years in a patient who had a major thoracic spine dislocation with paralysis. We took him to the operating room and repaired his thoracic spine. The surgery took 5 1/2 hours. Remarkably, he fully recovered from his paralysis but was left with partial visual loss.
ION (ischemic optic neuropathy) is very rare and precautions can be taken to reduce the potential, but the risk is there, especially with long cases (even open heart and other surgeries).