Likutei Moharan 18:7/ליקוטי מוהר"ן י״ח:ז׳

17. Now, the takhlit is called tekhelet (sky-blue wool). This is because tekhelet is a blending of black and white and is a color between black and white. When black and white are brought together—as in writing, which is the concept of black [ink] on top of white [paper] and where certainly the undersurface of the letter is attached and close to the paper which is white—in the interface and closeness the black and the white are blended into the concept of tekhelet .

2And the undersurface of the letter is the end and the goal. The reason for this is that when a scribe writes, [he begins] by placing the reed or the pen with the ink on the [surface of the] paper. Clearly, the ink is still somewhat removed from the paper until the scribe presses the pen with the ink into the paper’s fiber. Then the ink’s blackness becomes very deeply and closely attached to the fibers.

3We can conclude from this that the coming closer is the end and the takhlit, and in the place of the closeness the paper and the ink are blended together in the concept of tekhelet, as explained. For such is the matter: the takhlit, which is the end of the deed, was “first in thought” and close to the thought, as above.

4{In my humble opinion, this is the explanation: the end of the deed corresponds to the blackness, whereas the beginning of thought corresponds to the upper whiteness, as is known. Thus, the end of the deed is the takhlit, which was “first in thought” and closer to the thought. This is the concept of the tekhelet, which is a joining of black and white. Understand this.}