Some blind cave salamander have eye how might this be evidence that cave salamander evolved from sighted ancestors

Eyes would be a vestigial structure.

Explanation:

A vestigial structure as a stunted organ that has little or no function in a particular organism. An example of this can be seen in the question above which states that some blind salamdras have eyes. These eyes have no use for these salamanders, so we can say that the eyes of the salamander is a vestigial structure.

According to the theory of evolution, organisms have undergone and undergo changes over time, with new species emerging from some common ancestor. Studying different groups of organisms, we realize several characteristics that serve as evidence for this theory. The presence of trace structures is one such evidence.