There must be some problems for your eyes. Your eyes may get infection. In order to avoid it, you need to have the good rest for the eyes, not sitting in front of the computers or tvs for a long time with little rest. You should also keep the good diet to keep the balance of nutrition for the whole body. You could use the eye drops if you do not feel comfortable and feel dry at the eyes. If you do this, you could get rid of it.
If your eyes keep blacking out, you should pay attention to it because it indicates that there is something wrong with your health. There may be irregularities in your blood pressure, so you should have your blood pressure tested and do something about it. Aneamic and diabetes are also the cause of blacking out. When there is problem in the content of Sodium or Potasium in your blood, you will experience the blacking out. A rare form of migraine which is called basilar migraine can also cause blacking out, for it will shut down the basilar artery temporarily. If you suffer from the blacking out continually, you should see your doctor as soon as possible to get timely treatment.
You know, as the development of the society, a great number of people will get this symptom from different reasons. Somebody get it just because of overworking, others may be get it by don't have a regular bedtime. In those state, because your eyes couldn't get a good blood circulations that caused it became black. If you want to get rid of it, I suggest you pay attention to rest and keep a regular bedtime. The most importance is before going to sleep at night with hot towel to the area for a few minutes.