Took a glance through the archives, but I didn't see this addressed. Apologies if it's already been answered.
Nasir Meidan:
Whenever you encounter a piece of ice that was just rezzed, lose all credits in your credit pool. Gain credits equal to the rez cost of that ice.
Blackguard:
Whenever you expose a card, the Corp must rez it by paying its rez cost, if able.
Once per run, you may expose an unrezzed piece of ice when you approach it. You may then jack out.
Nasir initiates a run, uses Snitch to expose a piece of ice. Blackguard then forces the corp to rez that ice.
a) I'm assuming if Nasir opts to jack out using Snitch, nothing else happens, because he did not encounter said ice.
b) If he chooses to continue, does his ID ability trigger? My inclination is that it does, but I suppose it depends a bit on what "just rezzed" means. It seems like Nasir was written with step 2.3 in mind, and Snitch is firing (I believe) at step 2.