The DM has somewhat invented the situation, and has not limited their own activity to the core rules. A "merge with a dragon" is very much a home-brewed situation. Now the DM may have in their own mind an exact solution, but also it is quite common to just roll with an idea because it seems cool.
I think this kind of character change is often a story-level one, and the DM may well have a mental image of how the merge has worked, and the sorts of things that may give your character back some sense. In addition, they may even have some sense of where things are going, or a specific plot point where the problem is resolved one way or another. However, they may also be following a pattern of throwing you a hard in-character problem and seeing where you take it.
As such, at the very least you will need to run ideas past your DM - by either trying them, or by talking through beforehand to get a sense of how the DM is thinking about the situation. You won't get a guaranteed win from any spell, short of a Wish to gain eyes or remove the merge (it would be a hard-nosed DM indeed that would stop Wish or Miracle curing such an affliction).
It is worth trying to understand how the dragon became blinded in the first place. If you know that it will give you clues as to what could work. The suggestion of Regeneration spell is a good one, if for example the dragon had eyes originally, but lost them somehow.
Off the top of my head, the resolution could be:
Getting the merge reversed
Fixing the dragon's eyes
Somehow getting your eyes expressed in the merge
Gaining magical Blind Sight, Echolocation or other sense to make up for the blindness (this might be a cool option, and the image of a super-skilled warrior who cannot see is a common trope).
None of these are directly in the core rule book, they would come either from the DM's plot, or some combination of your suggestions for fixes and the DM's agreement on "that's how it works". It is worth trying some low-level spells such as Darkvision, simply because they are cheap to try, and will give you clues on how the DM is thinking. Don't worry too much if they don't work, in fact be prepared to find out that most simple things you can in character do not fix things.
Meanwhile a positive approach to play is to keep the character's blindness "on screen" so it is not forgotten (try not to nag or get depressed as a player about it). Try to have fun with it, at least from a story-telling perspective, and make it a key goal to pursue fixing it whenever possible - e.g. a stop in town should involve some research on what the options are.
Finally, if the change looks long term (no access to suitable magic, and DM not progressing your character's plot quickly to resolution), and you think that your character's new hybrid form is worth keeping then one simple thing you can do is gain Blind Fighting feat at the next available level for a feat. This may be worth it if the powers of your new hybrid form are fun to play, and if the DM has balanced the blindness in the merge somehow, this could even still be effective in combat (I'd hope at least you get a breath weapon, which usually doesn't need accurate aiming to score hits).