Yeah, they could've used something simpler and less hand-wavy. Irreversible climate change or a freaking asteroid on a collision course with earth and they needed the black hole data to build something to deviate it. Literally anything else would've worked better.
I think the blight was chosen as a metaphor for the virus of defeatism and anti-scientific attitude. To which the movie proposes (and arguably wants to be) a cure, made of optimistic sci-fi, cool science and, of course, love.
The issue on the literal level of the narrative is that if you have no way to control the infection on earth, there's no reason to suppose you won't bring it with you to the next planet.
