I'd assume donated blood matches the average level that people already have in them, so not sure it really matters. But if you donated regularly enough, you could be donating blood that has lower than average levels!

PFAS doesn't immediately harm you like missing a few pints of blood does.

And it's not like you're concentrating higher levels of PFAS into the recipient, they likely have the same average blood concentration levels as the donor does since we're all equally exposed to the same sources.