Yeah only real reason why Fire Blast is on top of HP Ground is because Probopass + Bastiodon usage is about 10% (a good estimate considering no one would use them on the same team), while Cryogonal on its own gathers 8.8% usage. Add Vileplume / Amoonguss usage to that (that's another 9% usage, with the same assumption because they occupy the same niche) and I'd say Fire Blast is just as important, if not more, than HP Ground. If nothing, Fire Blast gives it some super effective coverage and stuff.
Though, if I were to squeeze Dragon Pulse on the set I'd probably use it over Fire Blast, too, because Rest is kinda useful and you never wanna get fully walled by Probopass.
Well I made some calcs to back them up but instead of putting them here I might as well just put it in the OP.