I think dismissing Darkblast by comparing it to Disfigure and focusing on how little it hits straight-up is underestimating the value of repeatable, on-demand removal. You're not "giving up a draw" to dredge Darkblast; you're choosing to draw a repeatable source of removal. I think if it hits in the realm of 33% of your cube, that's a reasonably balanced card, given that you can draw it multiple times a match, on-demand. That's not even considering the fact that many times it will be used like a "combat trick", a rare thing to justify in many cubes but perfectly suitable in Darkblast. I mean, if your standard is 4 copies of Doom Blade, yes, Darkblast might not compete, but if you're trying to curate something more middle- or lower-powered, the card is a complete house that helps support an archetype (self-mill) that is extremely difficult to pull off without holistically re-shaping how card draw works as a mechanic in your list.