Of course, the metaphoric expression "to be blind" means to have sin. But, then why does the Lord say that "if you were blind you would not have sin"?
Does He deny that they are blind? Yes, but only apparently, for in reality He asserts their blindness (Matthew 15:14). So, what then? It seems to me, one should distinguish two sins: a) epistemological/cognitive sin of ignorance (for instance, a person who does not know that habit of overeating is sin of gluttony, for nobody has enlightened him on that); and b) the very habit and inclination that is harmful for soul, like gluttony, pornography addiction etc.). The gist of the matter must be in relation between a) and b), that is to say, the modality a) of having sin b), namely knowingly, or unknowingly or “blindly” for that matter. I shall try to elaborate below:
The Lord says that until in their conscience there is nothing that would tell them that they are wrong, they are both “blind”, that is to say, ignorant, and in a certain way also innocent through this very ignorance; but if they possess in their conscience a vision of something better and greater, but behave not according to the latter, then they are neither blind, nor innocent any more, for the absence of ignorance. For instance, if I do not betray my wife with another woman but have no problem with looking at her lustfully, I may have no sin, for being blind I think I commit no adultery by this, for for me, and even my for my wife, the semantics of adultery is limited to just betraying my wife with another woman. But when the Lord tells me that even to look at woman, even at my wife, lustfully is an adultery, then I possess a vision of a greater thing and, thus am no more blind; yet, being neither able nor willing to change my former habits, I perceive those same habits already as being sinful habits and thus myself as living in sin, since I abandon not those habits. In this case I might long even for the times when I was blind and ignorant of the Lord’s new commandment, for at that time of blindness there was no sin upon me, or, to be precise, sin was there, ontologically speaking, but no guilt on my part, for I had no possibility to recognize the sin as sin.
Thus, when the Lord says that “had you been blind, you would not have sin”, must mean that “had I not shown you things and perspectives that only I could have shown you, then your limited visions and corresponding behavior and habits would still be, from divine perspective, sinful, but you would not be guilty for that.” Yet now, even after having listened to the Lord, they shut the ears and eyes of their minds and asserted arrogantly that they were sufficient in their knowledge, not needing any further enlightenment, and thus, not blind. Had they humbly and wisely acknowledged their blindness, i.e. the insufficiency of their knowledge, then they would have opened for themselves a perspective for spiritual development through letting the Lord to enlighten them.
Thus here is a nuance and two things can be distinguished: 1. Sin is not recognized as sin because of ignorance/blindness and then there is no guilt, because a blind is not conscious of his blindness; and 2. A blind is given a new perspective by Lord and he knows already this new perspective, but he does not like it and that's why turns a blind eye to it, or asserts that this new perspective is wrong, and he is still on the right side, and thus, not blind; thus, being blind he does not acknowledge this blindness as blindness; yet, already there has crept a sin of recalcitrance in his soul, for he knows in the deep recesses of conscience that he has not accepted a greater, albeit uncomfortable, truth from the Lord.
And the bonus 3rd point: a good heart who understands that his previous mindset, before the Lord's new revelation, was not correct, acknowledges it as "blindness", but since this is not easy to overcome, he opens up to God's graceful acting in him and gradually overcomes it in this salvific process; thus a person in a salvific process of God's grace working in him can be called as "not having sin" for even if he has sin in this process, this process eventually will eliminate it. A simile: I acknowledge my addiction to alcohol as sin and am no more blind, but sin of addiction remains in me, and I am in a process of overcoming this addiction through help of the Lord. Uh, difficult! Because, can we ever say that we fully understand what divine perfection is? No. And if so, every faithful Christian should be ready to acknowledge "blindness", that is to say, acknowledge that our grasp of divine perfection is limited and always subject to enlightenment from Above, the Giver of Light, only so can we be subjects of infinite growth in Lord, which is His purpose for us.
But I am not fully satisfied with this answer of mine, feeling that there loom some more paradoxical insights in it.