It's not an unforgivable act per session, but it's often very deliberate.
Can you imagine any other 'sin' where you first have multiple discussions with your spouse about whether you'd do it or not, talk to your doctors, the elders....already had a signed will in your wallet saying that other people shouldn't force you commit this 'sin'.....and then go ahead and do it anyway? It would take a very bold and 'brazen' attitude to be so deliberate about 'sinning'
On top of that, accepting blood is not a fun way to break the rules. Getting drunk or having sex with your hot colleague seems much more fun and tempting.
And while both genitals and blood are considered to be sacred, I guess most JW don't really view their genitals as such :-D
Oh and other sins are not 'trying to save your life', and JW have been indoctrinated with Jesus' words that 'everyone who tries to save their life will lose it'.
So their is a huge mental barrier that stops JW from accepting blood.