All things considered, Fire Blast should do x2 damage against a Cubchoo, since it doesn't have Thick Fat.
It might have been holding an Occa Berry, in which case the berry would reduce the damage done by a super effective fire move once, and then the rest of the fire moves will do x2 damage.
EDIT: I just saw what Mew wrote, and that's right.
Flame Burst deals damage. All Pokémon (including allies and the user if applicable) adjacent to a Pokémon hit by Flame Burst in Double and Triple Battles will lose 1/16 of their maximum HP in typeless effect damage. This secondary effect will not occur if the target absorbs the primary attack with Flash Fire or the attack misses.
If it's Flame Burst, it does typeless damage, despite being a fire type move. If it's Flame Burst you're referring to (and not Fire Blast), then that was why it wasn't super-effective against a Cubchoo.