Many in healthcare have relegated Chiropractic to the role of Pain Management.  While irritation of Spinal Nerve Roots is painful and relieving that pressure can relieve pain there is more to it than that.  Chiropractors help the body run more efficiently.  The Nervous System after all is the chief control mechanism of the body.  When it is impaired the whole body suffers.  That impairment can open the door for dysfunction and disease.

One interesting note for those that don't know a whole lot about neurology.  The brainstem in some people actually protrudes through the hole at the bottom of the skull called the "Foramen Magnum."  That means a very important part of your "Central Nervous System" can become impinged by misalignments of the First Cervical Vertebra commonly called the "Atlas."  The brainstem controls many of the "Autonomic Processes" of the body which makes it very important. 

Cranial Nerves also come out of the brainstem.  Specifically Cranial Nerves 3, 4, and 6 can affect eye function.

Functionally, C1 is the vertebrae that holds the skull on the neck and functions alot like a washer.  It rotates around a peg that is formed by C2.  When C1 is misaligned it can put pressure not only on Spinal Nervous Roots but also part of the Brainstem and the Spinal Cord which is at it's widest in the whole body at this level. 

Adjustments aren't a universal treatment for blindness because blindness can have multiple causes.  However if the blindness results from a misaligned vertebrae putting pressure on a neurologic structure, than yes an adjustment can have a chance at curing it.  This is what appears to have happened here.   Migraine sufferers typically have visual changes associated with their problem also.  Often these are alleviated by having the C1 level adjusted.