It's really easy to memorize because you just need to remember that the three youngest members' blood types are B and Onew's is O and Jonghyun's is AB.
Jonghyun's blood type is AB,Taemin's blood type is B, Key's blood type is also B,Minho's blood type is also B same as Taemin and Key and Onew's blood type is O
I can't remember but the last time I saw something about their blood types, it said:
Onew was a type O
Jonghyun was a type AB I think
Key, Minho and Teamin I think were the same, I think they are a type B