You aren't. You're voting to increase the pool of people likely to die by one, and hoping for others to put themselves at risk to mitigate the consequences of that choice.
If the original scenario were the same except with the addendum that a single unknown person had been forced to vote blue, then voting blue or red would be more aligned with how the blue voters are imagining it right now. Red voters would be choosing to prioritise their own lives ahead of the death of another, and blue voters would be putting themselves at risk to save a life. In that case, I'd like to think I'd decide to vote blue. However, in a scenario where everyone has exactly the same choice, with no impediments, and one of those choices is to survive, all voting blue is doing is choosing to create a problem which only previously existed as a self-chosen ambiguity: you don't know if anyone else voted blue and decided to put themselves in danger, but you do know they made that choice of their own free will. By voting blue, however, you know for sure that there now exists a pool of people, even if just one, who will die if billions of others don't risk themselves by voting the same way.