"Speak to the children of Israel and you shall say to them that they shall make for themselves fringes on the corners of their garments, throughout their generations, and they shall affix a thread of sky blue [wool] on the fringe of each corner." (Numbers 15:38)

...the fringes are a symbol of our being servants of the L‑rd...

Now we shall try to understand the mystical dimension of why this whole commandment applies only to garments which are four-cornered. Inasmuch as the fringes are a symbol of our being servants of the L‑rd, G‑d commanded that the symbol should also testify as to Who is our Master. Other nations too have their symbols by means of which they can identify who is a servant of whom. G‑d decreed therefore that we must not attach these fringes to garments which are not four-cornered, so as to symbolize that we are the servants of the Master Whose Kingdom extends to the four corners of the globe. This symbolism would be lost if we attached fringes to three-cornered or rounded garments.

G‑d commanded that the threads be white to symbolize G‑d’s attributes of Mercy and goodness, something traditionally symbolized by the color white. The color blue symbolized G‑d’s mastery in the Celestial Regions, seeing the color blue is similar to the color of the sky.

The number of threads i.e. 8 or 4 folded over, also symbolize His Holy name of four letters. His uniqueness in His Sanctuary is equivalent to the number 8, i.e. a combination of two of His names, Y-H-V-H plus A-D-N-Y.

According to Menachot (39) it is a mystical dimension of the halacha that the knot with which the blue thread is tied together with the white threads symbolized the mystical dimensions of kindness and mercy respectively. This is the mystical dimension of what constitutes heaven, related to our patriarch Jacob who represents the attribute of tiferet, harmony.

...the blue thread winds around the white threads, i.e. mercy coils itself around kindness, love.

Kabbalists who have studied the mystical elements of the written Torah claim that the attribute of tiferet yearns for the attribute of chesed, kindness, the quality attributed to Abraham. This is the reason that the blue thread winds around the white threads, i.e. mercy coils itself around kindness, love. The relationship of the two is similar to the relationship of ruach to neshama.

I wanted to explain why G‑d did not permit any blue except that which is derived from a fish which comes out of the sea, and not from any other source which is itself blue. Perhaps this is what the Sages meant when they said in Chulin (89) that the sea resembles the sky.

We should also understand that mercy is rooted in the Torah which itself is allegorically compared to the sea.

[Selected with permission from the five-volume English edition of "Ohr HaChaim: the Torah Commentary of Rabbi Chaim Ben Attar" by Eliyahu Munk.]