system
Given the following post, respond as if you're commenting on Reddit:
human
[Interstellar] What was the Blight? Where did it come from?
gpt
The Blight was kind of intentionally undefined. What we know is that was a fungal or bacterial organism that thrived off of nitrogen, not oxygen like most life. Given that nitrogen is 80% of our atmosphere, it exploded. 

Where it came from is most likely just an unfortunate mutation. It most likely isn't a single strain, and instead one group that, due to an over abundance of nutrients (nitrogen) and exploding population, was given significant opportunity to mutate and overcome any biological defence mechanisms plants had. As the blight consumed the nitrogen in the air and the infected plants, it halted oxygen production. Given how much oxygen is also produced by microorganisms in the ocean, it's also likely decimated that ecosystem, either by out competing them, killing them directly, or starving them, though it's most likely the second one as a mass die off of the oceans complex organisms would probably sustain oxygen production and to starve all the fish in the ocean would take hundreds or even thousands of years.