I donate blood roughly twice a year, and have been doing so for 5 years now.
But I do not count not donating as killing - withholding help, possibly critical one, but not killing. Because it is their body, their blood. And while there could always be more donors, the situation (at least in my country) is rarely if ever so bad that someone dies decidedly from lack of replacement blood-products. So it's not like the act of not donating can be directly causally linked to dead people, because the rest of us are donating sufficiently.
But if someone does not donate despite the ease of doing so (nearby blood donor point or a circulating mobile unit is visiting) by basis of nothing but "not interested", I do see it as somewhat morally reprehensible. But at the same time, some people have risks associated with donating blood, are clearly frightened of it or it links to a phobia or various medicines they are on forbid it are all valid and understandable reasons in my mind.