If you miss on your first attack with Corpse Blast, does this mean you miss your familiar and it remains in play and you can cast Corpse Blast again or is it still defeated?

It's defeated. Basically it blew up and didn't hit anything. While we are on the subject of Necromancy tho, can someone tell me how Fury of Undeath is supposed to work? Is it a seperate activation of the Familiar?

Yes, basically you are sacrificing one of your actions to give the Reanimate a second activation.

If you initiate a corpse blast using the Reaper's Scythe does the necromancer gain health for each monster killed in the blast? Can the Necromancer kill the Reanimate with the Scythe to gain health?

1) Yes, when attacking with Reaper's Scythe, the Necromancer gains 1 Heart for every monster defeated in step 5 (Deal Damage) of the attack.

2) No, the Reanimate is not considered a friendly figure and as such it cannot be targeted in an attack. Even if the Reanimate would be defeated by additional effects within the attack (such as Blast) the Necromancer would still not gain 1 Heart, as the reanimate is not considered to be a monster.