Objection and IAR would agree with the application of expert belt to Blast Burn. I couldn't get a chance to speak to IAR the other day, but it was he who ran the ingame test for expert belt, which showed that expert belt applies to heatproof pokemon. Objection, who I just spoke with on IRC would support this...
Atharla imo, if using flamethrower on heatproof bronzong in-game gives the it's super effective message, then it is to all intents and purposes in ASB super effective
But anyways, it's your call and I respect whatever decision you make. :)
Yeah. Ingame precedence.
[.QUOTE=Bulbapedia]Heatproof halves the damage done by Fire-type attacks and the burn status ailment.
From the wording, we see that heatproof doesn't move the effectiveness of fire one level down, and thus fire attacks still strike super-effectively, meaning that expert belt is still applied.
I believe somebody tested this ingame (was it jesseus?) but I can't remember who. But since expert belt applies to heatproof Pokemon ingame, "in the lack of an ASB precedence we follow ingame precedence". On another note, there probably is an ASB precedence and if you'd like I can fetch it up for you.