Earlier on? Yes, I think it would've felt less jarring. Perhaps even a specific about gender, or even a sentence about physical appearance, or some casual remark about him being from a family with financial troubles (comparing him to Ron, even). One or two sentences back in PoA or even GoF might've lessened this impact of BLAISE!ZABINI! that we got in HBP.

I think I get what you're saying. It's not necessarily that what we got of Blaise now makes it hard to fill in the blanks so much as "Dude, he's been around for *five years*. Why are we only hearing this stuff about him *now*, all sudden-like?" And yeah, I get that. It feels like overcompensation, and it's annoying. Because there's no reason some of these details couldn't have been dropped to readers all along the way.