I noticed that the fringes (tzitzit) are blue, the blue stripes and star of David in the Israeli flag. And also Rabbi Gutman said in a youtube video that the color blue must not be seen while meditating.
What does this color symbolize ?
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@Danno is on track! See this, also. It's a bit more comprehensive.
Blue in Judaism is used to symbolise divinity, because blue is the color of the sky and sea. It can also represent equilibrium, since its hue suggests a shade midway between white and black, day and night.
According to several rabbinic sages, blue is the color of God’s Glory. Staring at this color aids in meditation, bringing us a glimpse of the “pavement of sapphire, like the very sky for purity,” which is a likeness of the Throne of God. Many items in the Mishkan, the portable sanctuary in the wilderness, such as the menorah, many of the vessels, and the Ark of the Covenant, were covered with blue cloth when transported from place to place.
My surmise on why you should not see the color blue while meditating is based on what is said in the 2nd par. above and the verse it alludes to.
Exodus 24:10-11 with Sefaria.com translation:
וַיִּרְא֕וּ אֵ֖ת אֱלֹהֵ֣י יִשְׂרָאֵ֑ל וְתַ֣חַת רַגְלָ֗יו כְּמַעֲשֵׂה֙ לִבְנַ֣ת הַסַּפִּ֔יר וּכְעֶ֥צֶם הַשָּׁמַ֖יִם לָטֹֽהַר׃ וְאֶל־אֲצִילֵי֙ בְּנֵ֣י יִשְׂרָאֵ֔ל לֹ֥א שָׁלַ֖ח יָד֑וֹ וַֽיֶּחֱזוּ֙ אֶת־הָ֣אֱלֹהִ֔ים וַיֹּאכְל֖וּ וַיִּשְׁתּֽוּ׃ (ס)
and they saw the God of Israel; and there was under His feet the like of a paved work of sapphire stone, and the like of the very heaven for clearness. And upon the nobles of the children of Israel He laid not His hand; and they beheld God, and did eat and drink.
Sapphire is bluish, and they were not supposed to look at a vision of G-d. See Rashba"m's explanation for more details.
See this article about the colors of the Israeli flag. It was designed to mimic the colors of the Ashkenazi tallit which was blue and white. (I guess those were the common tallit colors then. I usually see black and white "racing" stripes as the common color combo.) The blue color on the tallit was mentioned above re techelet.
The Talmud (menachos 43a) states,
R. Meir says: Why was techelet singled out from all the various colors? Techelet is similar to the sea, and the sea is similar to the heavens, and heavens to the Heavenly Throne, as it says: "Under His feet was the likeness of sapphire brickwork, and it was like the essence of heaven in purity." (Shemot 24:10) (translated text taken from http://www.kby.org.il/english/torat-yavneh/view.asp?id=3806)
Thus we see that blue brings our attention to God. Anything blue after that (like the flag) was just modeled after the blue of tzitzit.
In the Zohar in several places, blue (תכלת) is taken to be the attribute of divine severity or judgment, e.g.:
האי תכלת איהי דין
This blue is severity.
תכלת איהי תקיפא ולית מאן דשלטא עליה לחיין, דאיהי כרסייא דדינא לשריא ביה דינא תקיפא
Blue is potent, and there is no one who can overcome it to attain life, since it is the throne of judgment, on which harsh judgment is to dwell.
כל גוונין טבין לחלמא בר תכלת
All colours are good to see in a dream except blue.
The connection might that the word for blue, תכלת, bears a seeming relationship to the verb לכלות which means: to put an end to, to destroy. Also relevant might be a comment of Rashi saying that תכלת is connected to תכלה, the Aramaic word for bereavement:
עַל שֵׁם שִׁכּוּל בְּכוֹרוֹת — תַּרְגּוּמוׂ שֶׁל שִׁכּוּל תִּכְלָא וּמַכָּתָם הָיְתָה בַלַּיְלָה, וְכֵן צֶבַע הַתְּכֵלֶת דּוֹמֶה לָרָקִיעַ הַמַּשְׁחִיר לְעֵת עֶרֶב
an allusion to the bereavement (שכול) which the Egyptians suffered through the death of their firstborn, for the Targum (Aramaic) word corresponding to שכול, bereavement, is תכלה (similar in sound to תכלת).
The Rashi cited above is based on the Mechilta - but Onkelos says otherwise. Since the blue color of techeles is precisely supposed to be used to bring our meditative state to a condition known as Yirah - awe of God - I'm not sure why Rabbi Gutman states that it "must not be seen while meditating".