The Thoughtless Form is the ultimate-absolute form, which lies outside of any and all thoughts. It lies deep beneath the subconscious mind, which many people think that the subconscious mind is the absolute lowest part of the mind, when in all reality, the Thoughtless Form is.
The Thoughtless Form is believed to be the afterlife itself, where we come and go between this life and the next. It is a resting place. Some may have experienced it while in a deep state of relaxation or in a deep state of sleep. It is pitch black, and no physical body will manifest in it, even if it's your own dream body.
I've seen the Thoughtless Form! It is very pitch black, and believe it or not, it was actually quite relaxing and felt restful, rather than scary or horrifying.