For over a Thousand Years the Techeles is Absent

Rav Chaim Vital in Shaar HaKavvanot (Tzitizis, Drush 4) writes that Techeles represents HaShem's presence being clearly felt by all. This is why, he tells us, Techeles was only widely accessible during -- or close to -- the times when the Temple was standing. These were the times when Hashem's Presence among Israel was manifest for all to see. After the exile and its attendant hardships intensified, however, when HaShem's Presence among His people is less evident, Techeles became "hidden" as well.