One of the rules of the death note says that if someone makes the shinigami eyes deal they will have excellent vision regardless of their previous vision.
The rule in question, (listed under the Death Note Wiki as "How to Use: XXI"):
"Those with the eye power of the god of death will have the eyesight of over 3.6 in the human measurement, regardless of their original eyesight."
Could Light or any of the other characters in the series have cured a blind person with this rule? It seems to be a brilliant way to avoid suspicion and get a true ally to cure a blind person in exchange for half of their life and abuse their gratefulness to make them kill people for you, and who's going to suspect a blind man?