Donating blood/plasma is a good thing. Economic conditions in which poor people feel obligated to give blood just to make enough money, whilst rich people don’t feel this same pressure, are bad.
I don’t know how exactly private blood banks work (in plenty of countries blood banks are public and presumably non-profit), but regardless, I assume nobody can get blood transfusions if nobody donates. So until the political system is overhauled just keep donating? Your blood donations aren’t the root cause of capitalism