She could not tell whether her diamond earrings were beautiful. She had lost her eyesight for years/ had been blind for years.
In the second sentence, which phrase should I use, 'had lost her eyesight' or 'had been blind'? And I'd like to know whether they are interchangeable?
being blind focuses (no pun intended) on the state and you should express the time as a time span "for years" - hence:
She had been blind for years.
losing the eyesight happens at a certain point in time (unles explicitly expressed otherwise), so the time is described as singular event "(x) years ago:
She (had) lost her eyesight years ago.
Thus your first suggestion doesn't work:
She had lost her eyesight for years
This is a matter of lexical aspect, which you may read about here.
She had been blind for many years is acceptable.
Be is a stative verb: that is, it designates a state (in this case, blind) which continues indefinitely. It is thus non-telic: its sense does not include the notion of a change of state at the end of the action. Another category of non-telic verbs is activities like run and listen. Like statives, activities continue indefinitely without any change of state; they differ from statives in requiring continuous effort
For (in this context) designates a timespan during which the action of the verb occurs continuously. It works very well with non-telic verbs like be and run: "She had been blind for many years", "He ran for ten minutes".
She had lost her eyesight for many years is not acceptable.
Lose in this context is a telic verb, one which designates an ultimate change of state: in this case, a change from being sighted to being blind.
With telic verbs, for timespan has to be reinterpreted: since the verb designates a change of state rather than a continuous unchanging state, for is understood to mean that the action of the verb is continuously repeated throughout timespan. For instance, write with a Direct Object like novel is telic—it ends with a change of state, the novel coming into being—so if we say "She has written novels for ten years" this is understood to mean that during that ten years she wrote a novel, and then wrote another novel, and then another, and another . . .
But if we try to use a for phrase with lose one's eyesight, we end up with an absurdity: "She had lost her eyesight for many years" implies that she lost her eyesight, and then she lost it again, over and over for many years.
If you want to use lost her eyesight you must use a different time expression, one which locates the loss at a timepoint:
She had lost her eyesight many years before.
Beyond this, there is no grammatical reason for choosing one of these expressions over the other. Had been blind is perhaps a little blunter than had lost her eyesight, but the choice between them is a matter of style and tone.
"She had lost her eyesight for years" is not valid. A close alternative is "She had lost her eyesight years ago" and in that the "had" would normally be omitted.
The two forms "She had lost her eyesight years ago" and "She had been blind for years" have the same meaning.
