Blue Tsitsit

The Torah states a mitzva that upon the each of the four corners of a four cornered garment tsitsit should be added, including a thread of tekhelet (blue). Nowhere is there any mention in the Torah of a particular blue. So why doesn’t everybody who wears tsitsit use a blue thread?

The identity of what is acceptable to make the dye for the Techelet is found in the Oral Torah. The dye was made from the blood of the chilazon which was some kind of marine life. Over the years the identity of the chilazon has been lost. Nowadays there are scholars who are researching what it might be.