Kabbalah zu Bereschit 27:1

Darkness is a black fire that is strong in color, because no other color can change black. It is also a red fire that is strong in its appearance, as red is the most noticeable color. It is a green fire, which is solid in form, as all forms reach perfection by the green color. And it is a white fire, which is the basis of all other colors. Darkness is strong, because it contains all kinds of fires and it attacks the formlessness. Darkness is a fire that consists of four colors, and it is usually not a dark fire except when it attacks the formlessness. This is the secret of the verse: "his eyes were dim, so that he could not see, he called Esau..."(Genesis 27:1). The face of evil is darkness, so Isaac, who was kind to evil, to Esau, was then called 'darkness,' as he rested upon it so that it may attack it....

"And it came to pass, that when Isaac was old" (Beresheet 27:1). Rabbi Shimon said, It is written, "And Elohim called the light Day, and the darkness he called Night" (Beresheet 1:5). This verse had already been explained; nevertheless come and behold, All the actions that the Holy One, blessed be He, performs, are true, and in the secret of the upper world. And all the words of the Torah are words of faith and supernal mysteries, as they should.

Thus, it is written, "And Abraham was old, advanced in age" (Beresheet 24:1), that is, in the shining lights (of benevolences). And he is old, as it is written, "that shines ever more brightly until the height of noonday" (Mishlei 4:18). Therefore, "And Elohim called the light Day." "And the darkness he called Night." This is Isaac, who is dark, and gets to receive the night within him. Therefore, when he grew older, it is written, "And it came to pass, that when Isaac was old, and his eyes were dim, so that he could not see" (Beresheet 27:1). (he had become completely dark). Assuredly he had to be (totally) dark to cleave well to his grade.