Requirements to be a Blood Reaper: Friendly, Non-Legendary, and has the Demon subtype, according to the card. Must be summoned either by the Warlock or a Spawnpoint that the Warlock controls, according to the Battlegrounds rulebook.
Obviously the creature in question met all these conditions the first time. Rise again doesn't change the fact that it is friendly, non-legendary, or a Demon, and the Official Rules and Codex Supplement clarifies that the Mage is still summoning the creature when it gets Reanimated. Thus, it definitely meets all of the requirements. You can certainly have a Zombie Blood Reaper if you would like.
It should be noted that a [mwcard=FWQ09]Sectarus[/mwcard] + [mwcard=DNE02]Rise Again[/mwcard] combo might not work as well as you may be thinking though. Sectarus has to deal damage before you can cast the curse. If that damage kills the creature, then there is no creature to cast Rise Again onto. Rise Again must be both in place and revealed before the creature dies in order to reanimate it (although you can wait until after the Roll Dice step before you reveal, it must be revealed before you Apply Damage and Effects).