I think you are often right about the marginal utility of blood. However, it is worth noting that the Red Cross both pesters people to give blood (a lot, even if you request them directly not to multiple times), and that they offer rewards for blood -- usually a t-shirt or a hat, but recently I've been getting $5 gift cards. Obviously, this is not intended to directly indicate the worth of the blood, but these factors do indicate that bribery and coercion is alive and well.

EDIT: The FDA prohibits any gifts to blood donors in excess of $25 in cumulative value.

It is also worth noting that there is a thriving industry paying for blood plasma, which may indicate that certain types of blood donation are significantly more valuable than others (plasma are limited use, but can be given regardless of blood type).