i get these purple blobs that you are talking about all the time. the more i focus on them and try to controll them the m0ore vivid they become, becoming faster, bigger and sometimes more colours. if you will them to move you can cotroll them.
i acctualy dont think that these are hypnogogic imagery, although this is what most people assume when they first hear about it.
when you get them strong enough if you open your eyes you can see them with your eyes open, it helps if the room is dark.