I'm not so certain that the issue is the definition of the term normal, so much as it is the value (positive or negative) or association (healthy vs. unhealthy) the term implies in a medical or social context.
One of the most common reasons for blurry vision while wearing contacts is that your eyes are dry. Some people have a medical condition that causes dry eyes, called dry eye syndrome, but others' eyes are irritated by the contacts themselves.
Permanent blurring of vision due to cataracts can also result from changes to the lens due to excess blood sugar. Cataract surgery may be necessary to remove lenses that are clouded by the effects of diabetes and replace them with clear intraocular lenses (IOLs) to restore clear vision.