As found on the [bbc]:

The former president of Disney's Pixar Animation is a top notch animator, and as it turns out also has a condition known as [Aphantasia], wherein he cannot visualize imagery in this mind. Little research has ever been done on the condition, first described in 1880. Interestingly, many people who have Aphantasia seem to also be some of the best visual animators and artists in the industry.

Going off on a bit of a theoretical tangent here, I've also come across whats been referred to as "Astral Blindness", wherein people who purport to astrally project their minds become blind during these experiences. Instead they tend to experience things like feeling everything around them as though its vibrating, hearing it, or just knowing where things are as though from memory. Part of this has me speculate that Aphantasia may not necessarily be "blindness" perse, so much as the core conciousness of a persons mind centering around a different core sense other than sight for spacial or memory cognition. Its particularly interesting, combined with other phenomina like Synethesia, how cognitive perception may be effected with those who have visual impairments. Food for thought...

There's a video that describes more of it [here], which also suggests there may be some cognitive excercises that people can do to train the ability to visualize.