Remember, he is a half-blood.. He must have his personal reasons for refusing to take blood.
Yoon is half vampire, so it isn't necessary for him to drink blood. that's why he 'eats' essence from sex instead. in the panel, Jooyoung said that it has been difficult for Yoon to drink blood since he was a child, so that explains it. however, he needs to drink the blood of particular pure-blood vampire and finding that vampire is kinda difficult
Oh...it makes sense now xD thx
Hold up...so he could've gotten essence by drinking blood?....why was the solution so much easier???(╯°Д °)╯╧╧ Things aren't really adding up right....