*amused* I think possibly it's a combination of Blaise's mother being described as the seductressy kind of hot, (though Narcissa can certainly subtly work it in that Snape scene) and that we have very little else for Blaise. We know more stuff about Draco, so we can talk about his personality, whereas with Blaise most of what we have to work with is some presumption of pretty.

Also Blaise gets described as hard to please, et cetera, whereas I'm not sure Draco could play it cool enough to be described as same. Plus, one always sees Draco the fiendishly attractive silver-haired blond or the ratface. Having Blaise be hotter is a good way to strike a happy medium, while embracing Draco being attractive as well. Because certainly, if Harry had something bad to say about Draco's looks, he would've said it at length.

As for Blaise not being actively described as hot by Harry - I didn't expect him to be. Because Harry is very good at noticing the hot boys (to the point of being disturbing, in Tom Riddle's case) but he never notices that any contemporary of his is hot. Sure, older boy, sure, diary vision boy, sure, godfather in past, but never a contemporary. Too real? Too gay for JKR? Too likely to lead to touching boys' shiny hair?