Dont argue the rights and wrongs of their teaching, only it's inconsistency and it's lack of logic.
Ask:
Is it a "matter of conscience" to give blood? NO
Is it a "matter of conscience" to store your own blood for transfusion? NO
Is it a "matter of conscience" to give blood for the removal of fractions? NO
Is it a "matter of conscience" to accept blood fractions obtained by the above methods? YES