The thing about fireblast is that it never gets cast when it can be drained. It sits in your hand until they tap out at the end of your turn, sitting on a few life and thinking they're safe because you're tapped out too... then it smacks them. The card disadvantage is irrelevant because it is purely kill or complete desperation that drives it -- see Karvek's Spite

. What I don't like about shrapnel blast is that it doesn't conceal any information this way - either you have 2 untapped, or you don't. 2 untapped says I might have double bolt (6 dmg) or shrapnel blast (5) - no guessing game there, fewer chances for them to screw up.
I do like that blast doesn't have to be purely kill, and knocks out some creatures sligh otherwise has a bit of trouble with - Karn, Su-Chi, Exalted Angel, etc - though none of those creatures are commonly encountered.
As I said test it out but shrapnel blast has a tough older brother to contend with...