A very interesting question!
First, it's hard to answer a question "Why there's no X in the Torah" in general, but according to the WIKI (in Hebrew), our ancestors didn't name the colors but instead refer to the similar substances.
THe question is based on our "optical illusion" - כחול wasn't so important and wasn't in "wide usage" as it is to us. The connection of the "legendary" תכלת" with the color "blue" is very late and questionable. Also, the color is not natural, not common in everyday life (AKA Mishnah) and difficult to manufacture. So I don't see a serious reason to mention it in our sources.
This article (למה אין כחול בתנ"ך shows that the phenomenon is common to other cultures, incl. the Greeks, that didn't have a name for "blue".
I have two hints:
According to Even Shoshan dictionary, כחול as a color (blue) was used only in the contemporary Hebrew (hence the circle), in the Mishnaic Hebrew it does not refer to the color blue but to the substance (dyes) used to color the garments or for women's makeup.
That's also what the wiki says: "רוב האזכורים לצבעים במקרא הם בעיקר לשמות של חומרי צבע"
Interestingly, the word תכלת used for Tzitzis is not a color either, it stands for the dye also, as the Targum does not translate it as a color (like in "וְעֹרֹת אֵילִם מְאָדָּמִים" - "וּמַשְׁכֵי דְדִכְרֵי מְסַמְקֵי") but as of a substance -
"עַל־צִיצִת הַכָּנָף פְּתִיל תְּכֵלֶת׃" -
"חוּטָא דִתְכֶלְתָּא" - thread OF Techelet, not "A Tchelet thread"