IOW, “blindness” has no meaning except in relation to sight. And we can experience the fact that any degree of blindness, anything less than full sight, is an obstacle to the good that sight achieves for us, i,e, full sight is the perfection of the optical system, it’s the reason why the optical system exists and therefore is part and parcel of its essence.
The idea behind the term, “the knowledge of good and evil” is that this knowledge is only obtainable as we experience both, because only then can we identify good as good and evil as evil. Prior to that, everything in Eden was good, so good wasn’t a separate reality identifiable on its own-there was no real reason for the word “good” in fact. Now we can come to know evil, the result of disobedience of and separation from God, and learn to hate it-and turn towards the source of all good, God Himself. Both physical and moral evil teach us this fact, paralleling each other so to speak, so that we come to know good and evil directly, viscerally. Unless evil can be identified as something at odds with and opposed to God, then this knowledge brings no benefit, this life has no purpose; God may as well have prevented Adam from sinning to begin with or just stocked heaven with the elect and hell with the reprobate from the beginning, as Calvinists believe He eventually does anyway. But either of those would’ve involved God reserving free will from man or overriding or eliminating it.
In any case evil is not God’s intention, other than to allow us to experience it, to wallow in it for a time like the prodigal in the pigsty, to learn run from it.