1) since they block out the warp and such, could they disrupt the effect of synapse in tyranids? could they block out/negate some of the abilities tyrants get? (like warp field and such)
They probably could, but the Hive mind is powerful and I'd imagine the first instinct of a synapse-less 'nid near a pariah would be to kill them.
2) the geller fields in ship block out the warp so it doesnt affect the ship. could a ship without a geller field be fine in the warp as well if all its crew were pariahs? (now i know there would never be a ship composed entirely of pariahs/blanks. it's just hypothetical)
The HH short story book notes that despite being filled with Sisters of Silence the Black Ships still need a Geller field, so no.
3) if a blank were to walk up to the Emperor in his current state would it kill him? (again hypothetical)
They didn't when he was alive, so I doubt it.
4) are there any space marines that have blanks/pariahs in their ranks knowingly or not?
There's no reason not too, unless the blank/pariah gene messes with any of the organs, but that seems very unlikely.
Of course I can't be sure about any of my answers- blanks/pariahs are rather a vague thing, for example what is the differences between them? Are there different strengths of them? What would happen if a physic power was used to say throw a projectile at them from out of their blank range? Basically I don't think anyone knows very much about blanks and pariahs for certain.