It is permissible for two people who are suffering from a blood disorder to marry and it also permissible for them to have children as all orders are in Allah’s hands.
However, this is a world of means, cause and effect. If it is established by reliable means and doctors that the offsprings may be infected with the disorder, then it would be advisable for such people to exercise precaution.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai