[Note: This originally came out of a post on the Facebook group, which was then reversed in light of Gregory Lewis's expert comments. The substance of this article is his; any errors are mine.]
If no one donated blood, a lot of trauma/hemorrhage victims would die, and the world would be a lot worse off. The average unit of blood donated goes pretty far, in terms of the expected value of the good it does. However, when considering whether or not to donate, we need to evaluate the counterfactual difference our actions make. That is, rather than looking at the average donation, the relevant measurement is of the marginal donation. This brings up the following considerations:
1) As it stands, a substantial number of people already donate regularly, and will continue to do so whether or not the (comparatively tiny) EA community does too.
2) Truly life-or-death situations are a minority of transfusions, and these are pretty much already covered by the existing supply. In fact, hospitals almost always keep an emergency reserve of O- specifically for these cases, so it's very rare that someone directly dies for lack of compatible blood. Because a large number of transfusions/donations happen each day, and blood product can often be transported to different hospitals to meet local shortages, projected supplies are relatively easy to forecast within a given margin of error, so it is possible for hospitals to maintain this emergency supply to handle urgent cases.
Thus, the effect of an additional donation to the existing supply is to help cases where the patient wouldn't be directly saved from death, but a transfusion would improve the quality of their recovery. Nailing down exactly how many QALYs this typically adds is very difficult to track, and probably hasn't been done in a rigorous way. However, there is reason to believe this number is not that high.
In the UK, a unit of red blood cells (RBCs) costs about 120 pounds. While financial incentives don't translate seamlessly into extra donations, this is roughly this price at which more supply can be obtained, so it roughly reflects the medical field's impression of how valuable it would be to do more outreach per unit. Furthermore, the typical cutoff for whether to fund treatment is ~20,000 pounds/QALY, which is much less efficient than the ~130 pounds/QALY one can get by donating to the AMF. (For more detail on these numbers, see this guesstimate and this explanation of it.)
Thus, for blood donation to be anywhere near as effective as the AMF (in terms of paying 120 pounds/unit for more product), the medical field would have to be undervaluing the effectiveness of blood donations by 2 orders of magnitude. Despite the lack of rigorous calculations done in the literature, a collective miscalculation of this magnitude seems implausible given the feedback mechanisms which exist in medicine, not to mention the tacit knowledge hematologists have developed from making these tradeoffs.
The role of effective altruism is to look for, and seize upon, moral opportunities that have been unfairly passed over by society at large. GiveWell-recommended charities, for instance, may sometimes get positive comments from economists, but receive insufficient funding to fully exploit the ethical gold mine that is their cause area. In the case of blood donations, the medical field generally has ways of spotting and filling in the cheap and obvious ways to save more lives, so our time is better spent on causes that aren't being watched over as carefully.
That said, there are occasional cases where emergency supplies dwindle. When this happens, specific appeals are made, and in these cases it probably is effective to lend some helping hemoglobin. Less crucially, regular donors often drop out on holidays and during the winter (due to colds/flu), so if one is inclined to donate, those are the best times to do so.
By regular morals, I mean basic morals such as treating others how you like to be treated, ie. rules that you would be a bad person if you failed to abide by them. While I don't consider EA superorogatory, neither do I think that not practicing EA makes someone a bad person, thus, I wouldn't put it in the category of basic morals. (Actually, that is the standard I hold others to, for myself, I would consider it a moral failure if I didn't practice EA!) I think it actually is important to differentiate between basic and, let's say, more “advanced” morals because if people think that you consider them immoral, they will hate you. For instance, promoting EA as a basic moral that one is a “bad person” if she doesn't practice, will just result in backlash from people discovering EA. No one wants to be judged.
The point I was trying to make is that EAs should be aware of moral licensing, which means to give oneself an excuse to be less ethical in one department because you see yourself as being extra-moral in another. If there is a tradeoff between exercising basic morals and doing some high impact EA activity, I would go with the EA (assuming you are not actually creating harm, of course). For instance, I don't give blood because last time I did I was lightheaded for months. Besides decreasing my quality of life, it would also hurt by ability to do EA. I wouldn't say giving blood is an act of basic morality, but it still an altruistic action that few people can confidently say they are too important to consider doing. Do you not agree that if doing something good doesn't prevent you from doing something more high impact, than it would be morally preferable to do it? For instance, treating people with kindness... people shouldn't stop being kind to others because it won't result in some high global impact.