I have found some specific reasons for each one of the plagues that GOD brought upon Egypt. Each Plague was a measure to measure punishments for the Egyptians:
According to Midrash Tanchuma the first plague of blood was related to menstrual blood. The Egyptians prevented Jewish women from ritually immersing after they finished menstruating. This prevented a husband and wife from having marital relations. Besides a cruel disruption of family life, the Egyptians intended it as a form of forced birth control.
The Nile River might have been a prime location for ritual immersion. (That’s where the daughter of Pharaoh performed her ritual immersion when, according to the Midrash, she converted to Judaism). Perhaps this was G-d’s message. You prohibited Jewish women from immersing in the Nile after their menstrual blood stopped flowing, I will turn the Nile river into blood. Just as the Egyptians disrupted marital relations among the Jews, G-d disrupted the Egyptians entire way of life. According to the Midrash all water in Egypt turned to blood. So no Egyptian could have a drink of water, wash their clothes or take a bath.
According to the Midrash, the Jews were forced to fetch reptiles, so in the second plague, Egypt was overrun with frogs. The Egyptians tormented the Jews by forcing them to sweep dirt from the streets and markets. The third plague turned the dirt into lice which tormented the Egyptians. The Jews were put in mortal danger by being forced to fetch wild animals. (Some commentaries saw this as a public show where Jews were matched against wild animals). Therefore in the fourth plague, Egypt were overrun by wild animals. The Jews were forced to graze the livestock of Egyptians on far away hills and deserts, to keep them separated from their wives. In the fifth plague, Pestilence killed off the Egyptian livestock. The Jews were forced to pamper the bodies of the Egyptians. On the sixth plague, the skin of the Egyptians was plagued with boils from which nothing could soothe the pain. The Jews were forced to plant gardens, trees, and vineyards. All were destroyed by the seventh plague of hail.
And the reason for the Final One, death of the Firstborn, is well known - since the Egyptians oppressed the "Firstborn" of the GOD.
But one of the Ten Plagues was missing, what is the specific reason (the midah keneged midah reason) for the Plague of Darkness? How was the darkness related to the Egyptian oppression of the Israelites?