Both Uses of

taint
in
Don Quixote

They are, in short, peasants, plain homely people, without any taint of disreputable blood, and, as the saying is, old rusty Christians, but so rich that by their wealth and free-handed way of life they are coming by degrees to be considered gentlefolk by birth, and even by position; though the wealth and nobility they thought most of was having me for their daughter; and as they have no other child to make their heir, and are affectionate parents, I was one of the most indulged daughters that ever parents indulged.†
Chpt 1.27-28

taint = to spoil something so it is not desirable

for though it is true he mingles mercy with justice, still, seeing that the whole body of our nation is tainted and corrupt, he applies to it the cautery that burns rather than the salve that soothes;†
Chpt 2.65-66 *

tainted = spoiled or contaminated

Definitions:

(1) (taint)
to spoil something so it is not desirable -- as when bacteria contaminates a food; or as when a rumor makes people distrust a person
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

Much more rarely, taint is used in a non-negative way to refer to a trace of something.