One of my players is an Asamir and purposely drunk devil blood in order to join a demonic cult?
Would he Die?
Yes, eventually, unless he took one of the various forms of immortality.
Aasimar do not have any inherent revulsion to devil blood. Unless the devil it came from happened to be poisonous or acidic, he would be fine.
What Ipslore said.
Nothing in the Aasimar racial writeup indicates that consuming devil blood is poisonous or hazardous to them, so nothing would happen.
(I was wondering why this was in Conversions, then I caught the bit about him joining a cult ... aka coverting to a new religion. Clever.)
He'd probably get a tummy ache. Blood is very rough on the digestive system.