R. Yissocher Frand on the natural aspects of the first eight plagues (link):
In all of the plagues up to this point, the Almighty did not change the order of nature. This means that all prior plagues could to some extent be given some "natural" explanation. For example, the Nile being filled with blood could have been the result of pollution, a type of 'oil spill', etc.; frogs can congregate in one place, occasionally; and so forth. None of the first eight plagues represented a fundamental change to the order of nature.See also this post.
However, the ninth plague of Darkness did represent a change in the course of nature. Three days of consecutive darkness, was a miraculous departure from the natural day night cycle. Why now? The Medrash attributes it to a special Divine irritation with the king of Egypt.
This time Pharaoh did something that the Almighty would not tolerate. Pharaoh was cynical. He mocked the Jewish people. He treated Moshe Rabbeinu with derision. G-d has, so to speak, a special aversion to cynicism (leitzanus) and mockery. The trait of cynicism and the practice of acting with derision is abhorred by Heaven to such an extent that when Pharaoh engaged in that behavior, G-d increased the intensity of the plagues by changing the order of nature.