OK - so this isn't like the usual way of getting to boundless space which is through samatha but it's definiately boundless and space (not black or anything - just spacial awareness). The suttas say that sariputta attains boundless space by discerning all objects and finding there causes and hence discovering the way out which agrees with this approach.
So here's how I do it (well - done it - you don't need to do the initial stages after you've found the later)...
Firstly I label everything - as many new pieces of sensory information as possible like normal vipassana.
Now - it's natural for when labeling for the mind to label one object, drop out as it passes and label the next. However, I stick with reality as the object passes and follow the movement of focus onto the next object - labelling this.
As you label the flow of focus in between more then the mind detaches from it. It is a wave of focus which contains spacial awareness. It also may contain little nimmita which correspond to the sense (like proto-sense).
Now I switch concentration more and more to the flow of focus and less to the objects it rushes to. I do this until I am continually with the flow of focus (but I still am with the senses).
Now I specify my labelling to just the spacial movement and posistion of the focus. So my labelling is like: "round there", "through there", "over there" etc. etc. This has the effect of switching your attention purely to the focus and and not to nimmita or sense. This allows the focus to escape the senses and as it does so it starts to shoot out miles away. I also label these - like "out there" etc.
You can then do it more sammtha stlye and drop the momentary concentration into a continuous flow or go into a concentration on a point "lightyears" away (hard though - the space needs to be pretty pure to do this.)