ExploderWizard
Hero
You don't see an issue when one of the players in a campaign is saying his enjoyment is being diminished? Isn't the point of getting together and playing these games to have fun?
The OP feels his character is useless in the role he is supposed to shine in because of a situation the DM has placed them in, not just for one encounter or two, not just one session even, but for an extended period of time covering multiple game sessions. His character tried to overcome the handicap but was told he couldn't.
The whole notion of " I am supposed to shine" can jump off a cliff. As an adventurer, sometimes you will kick ass, and sometimes you will struggle just to survive.
Whatever comes up, just do the best that you can with what you've got.
Diminished enjoyment is totally up to the player. You can have a sour outlook on any setback and choose to be surly or choose to enjoy the high level of challenge. Isn't winning through in the end even facing impossible odds what heroes are all about?
This character is supposed to be a ninja. For a ninja, being unable to see is only a mild inconvenience. Sight is only one sense. In the underdark it may be one of the least valuable. Echos travel quite a ways underground warning of potential danger long before it could be seen even with 120' darkvision.
Smell is also important. Smelling/tasting an area of bad air before stumbling right into it is important.
Be a ninja. Remember KAI and let the other senses do the heavy lifting when vision isn't the most reliable.