Risk of blindness from orbital decompression?

The actual percentage would be 0.2%.  Generally, to recommend orbital decompression, the benefits should out weight the risk.  The risk profile varies with the severity of the orbitopathy.  The risk of visual loss is significant but the number is not in the literature.  The reason is that surgeons have a hard time being that brutally honest about the work they are doing.  It is just not a practice builder.  I will get hot emails from colleagues who are in the business of offering cosmetic orbital decompressions which in many cases is just removal of orbital fat.  The risk for that type of procedure is definitely lower and most likely on par with eyelid surgery.   For that type of procedure, the risk of visual loss might be as low as 1 case in 30,000.  Deep orbital decompression for more advanced thyroid eye disease, deep orbital bone must be removed in addition to orbital fat.  These procedures have a substantially higher risk of visual loss, which in my opinion is not accurately described in the medical literature.  In more advanced cases, the individual is loosing vision due to the disease. That is why orbital decompression is performed. The risk is warranted in an effort to save vision from certain loss. This is also why research for a medical cure for thyroid disease is so important. 

The chance of blindness is extremely low, much lower than 0.5%. There is higher chance of double vision but that is very variable depending on surgeon's expertise, severity of thyroid eye disease, etc. See following link.