I agree with Amyjay. Your therapist probably need to know more. When that's happened to me it's because an alter pushes hard fronting, which pushes me out. I find myself either in a black void or I lose consciousness. Usually when they push me into the void there's a white spherical light source which is the alter who's fronting, but that's only when I'm also close to the front. Otherwise I just lose consciousness. The only other time I've had my vision go black is when a teacher pushed so hard on my chest that my vision started to collapse into a tunnel.