About 10 years ago I got my first visual migraine and I was completely stumped on what it was. At first I just thought that what I was seeing was from looking directly at a down light in the stair well of the office building I worked in. Soon after I got back to my desk I noticed that not only did I have the spot where it looked like I was looking through a kaleidoscope but I also could not seem to read words on my computer.
After talking to an optometrist about it I was informed that it wasn't anything to do with the eyes but just an "ophthalmic migraine" or "ocular migraine" other terms "visual migraine". These can be brought on by a number of things including food allergies. I've discounted the food allergy as a source for me by noting what I've eaten and when before I had an onset. So I'm still not sure what triggers them off though. They can happen any time of day for me too.
For those that don't know anything about them here is a you video that is similar to what happens to me with a few differences.
For me there is no real distinct blurring. The rainbow kaleidoscope effect is much more pronounced. They always start on my left side of my field of view and as they grow larger they actually engulf a larger portion of my view in the centre while spreading out.
The last thing took me about 7-8 years to figure out. I get a blind spot in the centre of my view. Now this isn't like a black spot but much more like the actual blind spot everyone has due to the optic nerve. We don't see this blind spot because the brain is so good at filling it in, Google "blind spot" or go here to show you what the blind spot is and how the brain fills in the info. Anyway it seems my brain is doing this same thing for my centre of vision, disregarding any into actually sent to it by the receptors that are actually located there. This has a very interesting and definable effect. It means it is almost impossible for me to read while still appearing to be able to see letters. The letter(s) at the centre of my view disappear in a way that isn't very obvious. I still see letters to each side but as my focus goes to them they to disappear or get altered. The result is much like trying to read a random set of letters for me.
Needless to say my visual migraines are a pain, literally as they most often progress to a normal migraine soon after they start, but they are interesting from the point of view of what is happening to the brain and what the brain is capable of doing. I can now remotely relate to some people with visual synesthesia but I don't get it from the cool things that other people get their visions.