Incorrect. It indicates continuing action, as opposed to continual. The difference may seem slight, but still is significant.
I once again checked, triple checked, and I would have to disagree with you. Intransitive verbs can indeed be middle or passive. As for a deponent, it is morphogically a middle, the active voice having 'deposited' or disappeared thus the word 'deponent.' Two issues then, can an intransitive verb be a middle or passive, the answer is yes; secondly, must a deponent ALWAYS be understood with the active voice, the answer is no. As for the middle voice, it often denotes the subject acting on or for itself.
But as such, it does not always have to be understood in the active voice.
However, it could as well mean that he blinded himself. The grammar would support that. In fact, the verse in question is silent as to the ORIGIN of the Jews' inability to believe, other than two factors, one the possibility presented by the inflected meaning, and second the immediate context.
The verb itself does not indicate that. It simply represents a continuing action, initiated by the individuals.
However, the verse does not say that. It would be an assumption. Context would have significance on that point. Nor does the verse describe a STATE of being, as in blind from birth, a one-time event, such as aorist would convey. It conveys continuing action in the past, hence the imperfect.
Depending on the message he wanted to convey. If he wanted to convey that they had been made incapable by an outside source, for example God, then the aorist would be quite appropriate. The verb then would describe a STATE of being blind. However, the tense and voice chosen indicate continuing ACTION in the past, initiated and participated in by the group described. If John were to want to convey the understanding that the group caused their own inability, and was describing the STATE of inability, then the middle voice would be appropriate, and the aorist would be a good fit. If he wanted to convey that the group caused their own inability, initiated their inability, and continued a course of action, then the imperfect would be a good fit. That is consistent with both the immediate context and the prophetic context as referenced from Isaiah. And those factors are part of exegesis as well.
Dave