Thank you everyone that took the time to answer!
From this quote by Darwins Finch in the thread he linked to I get the impression that Darwins proposed explanations (i.e. environmental triggers during the life of the individual animal; Lamarckian inheritance already being ruled out) has been disproven, at least for certain kinds of fish:
I also personally find Darwins Finch argument for pleiotropy being the cause as the most plausible suggestion.
Wouldn’t genetic drift produce in a species a diversity of different kinds of unusable visual systems as they degenerate in different directions with passing generations, under no selective pressure?
I also thought this suggestion by Tuckerfan was interesting as a possible disadvantage and cause for negative selection, though no-one followed up on it in that thread.
If you would rephrase that as this: eyesight, as all senses, constitutes an evolutionary advantage only as input to a system (i.e. in animals usually the nervous system) that controls the individuals behaviour in a way that increases the chance of survival and to procreate. Maybe feeding the control system with inputs that it was not originally adapted for (i.e. lack of sensory input) in some way degrades the proper functioning of the control system and therefore constitutes a disadvantage to the individual?