The foveae (or blind spot) are manaless astral tornadoes. They are commonly found in Aztlan believed to be a side-effect of the pervasive blood magic used in that country. Foveae can make traveling in Aztlan's astral space very dangerous. Awaken critters and spirits seem to be able to sense foveae and attempt to avoid them at all costs.
Unlike normal mana voids, there can still be a lot of mundane life in a foveae, and they appear to be a dark tornado in astral space.
It seems like in older Shadowrun editions (before 5th) foveae were just a synonym for manavoids, but as of SR5 they appear to be an astral phenomena. Possibly showing a greater understanding of the nature of foveae.