On the live servers, the trigger mobs Blood of Ssraeshza (for Emperor Ssraeshza) and Rhozth Ssrakezh (for Vyzh`dra the Cursed) appear to only spawn on server reset or after a patch. After an initial zone clearing and successful kills through to the end of both events, they do not respawn. A zone reset or patch day appears to be the only methods to guarantee a respawn of these mobs.
Blood is a random/timed spawn and will not always be up, it may only appear after the respawn lockout is up (and if it's killed before the Emperor is back up), so it's not always going to be up. Also, according to comments on popular user guides, the Emperor event will still trigger even if the other golem is up.