To quote 1d4Chan - "Things to do when you are in warp":
If you're a Null... How the f@#k did you manage that? Oh well, you're pretty much invincible. Have fun, troll away.
Keep in mind that anything you touch or even approach (depending on your power) would dissolve into nothing. This includes the things you're standing on, like floors, bridges and stairs. This may even include air - so while psykers and even regular humans can get away by believing there is air around and warp being twisted by their will to manifest that believes you're stuck with what you brought with you from the realspace.
The answer to this question is similar to the answer for "Can a man who never dreamed enter the dreamland?". Difficult, that is:
Lets first answer few basic questions:
Warp is "an adaptation of Carl Jung's concept of the Collective Unconscious, a conceptual Metaphysical dimension created by the common thoughts present in each sapient lifeform". In other words, nothing physical exists there, just like nothing physical exists in your dream. You can dream that you hit your head and wake up with a headache, but it is your body responding to what your mind told you (or other way around: you might go to sleep with a headache and dream about being hit in the head).
Demons don't have physical forms. To enter reality they need to be summoned, which creates a weak spot in reality and allows them to construct body from the "reality stuff" using their warp powers (via possession), or creating "warp bubble" around themselves, that allows them to exist in the realspace. They are really resilient to physical attacks that in fact only banish them back to the Immaterium.
Similarly as demons coming to our world, ships enter the warp by creating a powerful field that creates a bubble of reality around them. So we have a real ship (with its content, which can include Blanks) entering Immaterium.
Alive Soul possessing beings (which include certain inanimated object that posses Machine Spirit) can enter the warp directly, which is of course extremely dangerous (unless you are a Chaos Champion, that is). When they do, they stop existing physically and became their immaterial counterpart, just as in real space theirs soul don't exist "physically" but are somehow part of your physical body (sort of...)
Blanks don't have the immaterial representation of them. If somehow they would get into the Warp, probably one of the three things could happen:
a) Blank stops existing: without the immaterial counterpart there is no "immaterial" representation of him/her.
b) Warp spits him back immediately
c) He/she enter a warp has a small bubble of reality around him, with its size depending on the blank's power. Analogical to demon in the real world, he would be almost invulnerable, but not omnipotent - he couldn't be damaged by pure magic but he still could be "banished" back to the realsapce.