Before KMW, the Blood Curse was tied to what clan a vampire was. Every Assamite had the Blood Curse even if card text didn't say so (unless card text explicitly overrode that, of course). Now, as of KMW, the Blood Curse only affects the vampires whose card text say so (just like all of the other attributes: Scarce, Cold Iron Vulnerability, Sterile, etc.).

So Assamite Embraces and vampires that Clan Impersonate an Assamite don't acquire the Curse, and an Assamite Impersonating another clan won't lose it.