This is just a Mids bug. Some of the epic attacks for scrappers/stalkers have an incorrect average damage display. Specifically, it adds all the separate crit damage chances together, including the guaranteed hidden crit (yes, even though scrappers don't get that), giving a number that is much too high. Mousing over the damage breaks it into its components, so for Fire Blast we see:
62.56 fire damage (this is the normal hit)
4x6.26 ticks, 80% chance
62.56 fire damage (5% chance, this is the minion crit chance)
62.56 fire damage (10% crit chance, the Lt+ crit chance)
62.56 fire damage (to Critters, this is the hidden crit)
Then Mids adds those up: 62.56+4*6.26*.8+62.56*.05+62.56*.1+62.56=154.5
But really, against Lt+, the average damage should be: 62.56+62.56*.1+4*6.26*.8=88.8
Or against a minion: 62.56+62.56*.05+4*6.26*.8=85.7
They're decent attacks, especially Fireball, but they don't outclass the best stuff in your primary.
Edit: Technically the math for the DoT is also a little more complicated than how Mids does it, but that's a much smaller difference.
Are the numbers in Mids for Scrappers' ancillary pools correct? Specifically for Blaze Mastery's fire blast and fire ball? Because the numbers for Fire blast make it seem considerably better than most of the attacks in my primary (155 base damage at level 50 with a 1.2 s cast and 6 second recharge).
I'm trying to work on another high-end build and if those numbers are accurate, I want it in my attack chain.
Edit: I just noticed similar numbers with soul mastery's dark blast.