When an event card's special ability area calls for spawning genestealers (e.g., "Choose a Space Marine.  Spawn 2 Genestealers behind him."), are the genestealers drawn from one of the blip decks or do they come from the Genestealers deck? 

From my reading of the rules, they come from one of the blip decks.  The particular blip deck is determined by which side the Genestealers are placed to comply with the event card's directions.

However, if both blip decks are empty (last location card played), does that mean no Genestealers are spawned? 

Marximus said:

From the blip pile; you read it correctly. If if that blip pile is empty, then none are spawned (and the marines will travel to a new location at the end of that phase). You won't draw from the other blip pile though. See the top of page 22.

-Tim

Tim has it right that you draw from the blip pile, but I'd add one thing.  Page 14 uses exactly this event card as an example.  In addition to what Tim said they specifically state that a player must choose a valid target for event card abilities if possible.  Using your card as an example they state that a player must choose a marine that would spawn the most genestealers possible (two in this case).  So you can't choose a marine that would result in spawning only one genestealer when the you have another eligible marine that would spawn two.

lexlloyd said:

Second that.

If you have a left blip pile of 1 and a right blip pile of 4, and you have to spawn 2 GS, then you have to spawn on a SM's right as choosing the left pile would only end up in spawning 1 GS. If both piles would be 1 thick, you could choose both, only spawning 1 (sadly enough :p) and then offcourse travel at the end of the event phase (as one pile is now 0 thick).

When resolving :"Choose a Space Marine. Spawn 2 Genestealers behind him.", I always took them from the Genestealers deck. Where exactly in the rules does it mentions that they come from blip piles?

Brynjolf Irontooth said:

"Whenever a Genestealer is spawned, the current
player takes the top card of a blip pile and places it
face up in the appropriate position and side of the
formation." I found it. But I don't think that it means you would have to place them absolutely on a SM's right.
 

Burferking said:

I think you want to re-read the example + rule on page 14. If the right blip pile has 2+ GS and the left blip pile 0-1, you have to spawn from the right blip pile. Only case you wouldn't is if all the marines face right (and you need to spawn behind a marine).