Ultimately Altruism can only develop if the act of altruism by an individual is as big as the gain that individual gets in return... One example of this is mutualism. Cleaner fish set up cleaner shops to clean large fish of any parasites. This is highly dangerous because at any time the big fish could decide to eat the cleaner fish. But these large fish are recieving the benefit of having less harmful parasites and the cleaner fish are getting a meal in exchange. So ultimately it works. As for the example of donating to a blood bank it could be said that a person does it to just be nice, however one could argue that ultimately a person may at some point also need the benefit of having someone else's blood. Dawkins talks of the Cave theory and the never break ranks theory which at first glance seem altruistic by putting a warning individual at risk of being found by predators, when it really is quite selfishly just trying to keep other individuals quite so he is not found. Stotting also at first glance seems to be altruistic by diverting predators attention away from the group but really says to the predator, " I am not an easy meal, I am strong and will fight hard for my life, pick an easier target!" Symbiotic relationships also could have developed altruistic behavior.
In a simple game of prisoner's dilemma, there is no way of ensuring trust so it does not lead to altruism only best strategy which is both defect. In an Iterated or more repeated gamme of prisoner's dilemma repeated indefinitely, there is opportunity to build trust and to give that back. Tit for tat strategy in that you trust your opponent but do to him what he does to you in the previous round. Defectors are punished so to teach them a lesson. Looking at the British versus the Germans and their live and let live strategy, both had something to gain in this situation... their lives. As for the Vampire bats, their lives hang in the balance at any given moment when a meal is not available, so they share as they know they will be ultimately share with when theirtime of need comes, and the benefit to the one individual is much greater than the cost to the donator so it works!