Brain blackout is kind of a dramatic word. I'm pretty sure the article is trying to refer to cortical spreading depression.

https://www.nature.com/articles/nrneurol.2013.192

This is a wave of decreased activity going across the brain. It's not the whole brain though, just a portion, and it tends to happen more often in the posterior brain than anterior. That's why visual and other sensory auras (posterior brain) auras are more common than motor/weakness auras (anterior brain). The visual aura itself is the spreading wave of decreased activity going across the brain. It happens in primary visual cortex, primarily dealing with lines and colors. Visual space is represented radially on the brain, so it can often be circular. The "fortifications" or lines on the edges some people see come from the fact that it's neurons that deal with line detection. Pain usually follows shortly after, but we aren't exactly sure how that works, and this article was posing a possible mechanism to help link these. The main bulk of the visual aura where it's grey, blurry, and indistinct is the decreased activity itself in the visual cortex. The area can get larger as the wave spreads.

Deja vu or jamais vu have been reported with migraines, though that's a very rare aura in comparison. It's all depending on what parts of the brain are involved with the cortical spreading depression for that migraine aura for that person in terms of what symptom will happen. Deja vu would be more temporal lobe. Temporal lobe is the most common localization for focal epilepsies. So deja vu as a symptom of a neurologic disease would more commonly be seen with seizures (focal seizures are sometimes called auras too, which gets confusing but are inherently different from what is happening in a migraine). But don't worry, most deja vu is nothing to worry about.