Dragon's Breath has almost double the guaranteed damage (80 vs 45) and while there are a lot of enemies with an extreme weakness to Blight damage, there are also a whole lot of enemies with a serious resistance to that damage including demons, who are much more likely to be the sorts of enemies we're going to be dealing with at the point in the game that Dragon's Breath is usable. Those same demons also resist Fire damage, but not nearly as much as they laugh at Blight damage.
So you're looking at one weaker attack that is situationally either amazing or nearly useless with a bonus healing effect, and a stronger attack that's less likely to have a major damage swing due to resistance/weakness and it has a bonus that it will always do at least half of its expected damage. I'd say that balances out, and that's assuming no relevant tweaks happen between now and when Level 10 is attainable.