Say an ear infection from sticking a qtip in there to unjam something.
If an infection ensues, can it lead to blindness?
I reckon any untreated infection such as strep could spread to the brain and so on.
When my kid got ear tubes around age 3.5 he told me “Mommy I can see better.” I guess because he didn’t have to concentrate so much on understanding sounds and voices.
It’s a possible but extremely unlikely outcome.
Well, if my ears fall off, I won’t be able to wear my glasses, and my hat will fall over my eyes. So, yeah. 
I would have thought deafness was a more likely outcome, though.
A less remote possibility is damage to the sixth cranial nerve, which innervates the extra ocular muscles that allow the eyes to turn out or look to the side (laterally). This won’t cause blindness but will cause ocular motility restriction on the side of the infection.
I knew that. 
Can ear infections lead to blindness? Well, only if you masturbate during the infection.
I just learned that orbital apex syndrome (which can cause visual loss in addition to the more common paralysis of the eyeball) can result from ear and sinus infections (cites on request). So, another rare possible link, as Qad alluded.