Asslaamau alaikum brother nyekareem
Sorry I do not understand this as a possible reason for the blood tests before marrisgae. What you are talking about is not blood groups but DNA - genetics - and if two people closely related, i.e with too similar DNA have a child, there is a high risk of the child having some kind of disorder or illness or disability ( I dont think 'retarded' is a very acceptable word to use nowadays BTW)
But if this were the case - surely a couple when they went to get married would know if they were related to each other in some kind of way already?? the only way they would not know is if they lived in a village where cousin marriage etc happened alot and by some kind of connection they ended up being related and not knowing it - this is not likely to happen in a city
Plus the other thing I dont get is that even if a blood test showed up they had related DNA ( cousins or something) its no guarantee that they would have a child with a problem = many don't as we know. Its all the Will of Allah swt and He has allowed us to marry our first cousins so issues like DNA should not be our concern - if he gives the couple a healthy child alhamdulillah and if not - well then alhamdulillah for the child still and any problem they have is a test from Allah swt. I dont think trying to second guess Allah swt's will through inconclusive vblood tests is Islamic or accurate and I am not sure its the reason here as mentioned in the first post ( though of course if you know otherwise for sure please say and my apologies)
Forgive me I am coming at this from the UK angle where this is not heard of or a requirewment for marriage ( unless of course the couple choose to do it personally) so I am surprised there are mosques insisiting on this. Does anyone who goes to one of these mosques know the reason - ie have they spoken to the Imam about it?? Or is it just US Law in some States and its just something the mosques are being made to do??
Salaams