Is there a medical basis for left eye blinking superstition? A blinking left eye is supposed to be bad.

Superstition. There is no medical basis for that superstition, just like many other superstitions. There are several causes of eye blinking including blepharospasm, myokymia, foreign body, dry eye, and other causes. If artificial tears don't resolve it, please see your eye md.
Medical superstition. There is no medical basis for superstitions. I f that happened, a belief would become scientific knowledge. It happened many times in the history of medicine (what was believed to be spirits was viruses in some cases, etc). But no, not in the case of the left eye blinking (i never heard of it after living in different cultures). Thanks for your interesting question.