Blast Zone is an absolute beast in box stock form.  My Nephew loved being him in our Battle Tournaments because he could almost always win with just Bomb Throw, unless of course he was facing Dark Blast Zone!

I never understood why I didn't hear more chatter about him...I figured the Upgrade Paths must have been lackluster for him to be that ignored in chatter amongst Portal Masters.

However, after upgrading him I know that can not be the case.  Blast Zone has some great Upgrade Paths.

Up top, I think the obvious choice for most people is the Ignition Path.  This takes the already potent Bomb Throw and further enhances it.  Only catch is you have to have a fire wall to throw through.  This is the best choice for most people.

I prefer Reaction Satisfaction.  It might actually  be easier for rookie players to master as well.  Flame Breath, his tertiary attack, was left untouched in the base upgrades.  Reaction Satisfaction allows you to complete a circle and be rewarded with an explosion.  Phase 2 of the path increases both damage and area.  This to me is the best option not only because it does great damage, but because I felt it balanced him given how potent Bomb Throw was with just Power Bombs factored in...

On the bottom end I prefer Temperatures Rising.  It is a path that generates a fiery aura after dashing that allows you to drift towards enemies you want to target and do additional damage past your other attacks.  In other words, I feel it is a bit more precise than Fuel Injected.

Fuel Injected, however, hits much harder on the face value than Temperatures Rising.  At the end of  a dash you shoot off a fireball that does big damage to enemies.  The only issue is the lack of control.  You may want it to cascade towards a geargolem, but if a Greeble sneaks up from the side, it can auto-lock on the less than formidable enemy and fail to deliver like you had hoped.  That said, it is a minor issue and I think most will prefer Fuel Injected.

That said, any upgrade path for Blast Zone is well done and easy to follow, I just personally prefer Reaction Satisfaction for the balance and Temperatures Rising for the control.