Henry Alford commented,
The man shrewdly evades the inference and states again the simple fact. Bear in mind, that ὤν must here be strictly kept to its present sense, as being joined with a present verb βλέπω: the rule for the construction of a pres. part. being, that it is contemporaneous with the verb which rules the time of the sentence. So that we must render, not ‘whereas I WAS blind, now I see,’ as E. V.: but as A.V.R., being a blind man [or, though a blind man], now I see. The shrewd and naïve disposition of the man furnishes the key to the ænigmatical expression. He puts it to them as the problem, the fact of which he knows for certain but the reason of which it was for them to solve, that he, whom they all knew as a blind man, now saw. So that the ὤν carries not so much present matter of fact, as common designation and title.
As to the rule that Alford mentioned, Funk wrote,
849 Present and aorist participles: 'tense.' The tenses of the participle do not, strictly speaking, represent time. The temporal element is derived from the relation of the participle to the main verb (§846.2).
849 (3) The present participle usually represents action in progress at the same time as the action of the main verb. The simultaneous participial action may thus be in the past, present, or future, depending on the tense of the main verb.
In other words, if the participle is in the present tense and the main verb is in the present tense, we should expect to translate the participle into English as a present tense with the action being contemporaneous with the action of the finite (main) verb. The KJV translated it as though the main (finite) verb was in the aorist (rather than present) tense.
Show thyself approved. :)