(Will reply here for both Percy and Ernie to save space :-p)
Those were fun! Even when playing poor Percy's reaction to getting dumped for Ernie :-p
Blaise & Ernie: Definitely up for seeing what sticks, though I think Blaise is also smart enough not to piss off one of the Duelling club healers whose job it would be to fix him if he's hit by a curse (especially seeing as Ernie's specialty is spell damage)
Blaise & Percy: Would def. know each other, might even get on well - though be wary, the friendlier you are, the more likely you'll be to be 'forced' to look at baby pictures. And you're right, no matter how drunk, I don't think Percy would have shagged someone that underage - but it's okay, we'll still have fun!
Other: I'm actually working on a 3rd character, older Slytherin who graduated at the end of Blaise's first year, so there may be some place for connections once she's done ;)