The correct words are "Hashem Yikom Damav". We refer to the murder of one person as "Shefichas Damim" , the blood being plural.
Update:
I just saw in the daf yomi that there is discussion about why the Torah uses the plural regarding the blood of Hevel (Abel) when he was killed by Cain. One answer given is because when one is killed, all his potential future descendants are also "killed". Hence, Hashem Yikom Damav, G-d shall avenge his "bloods". That is why we refer to murder as "shefichas damim".