On his deathbed, Yaakov calls his sons together to reveal what is to befall them in the days to come. To his son, Zevulun he portends:
“Zevulun shall dwell by the seashore;
He shall be a haven for ships,
And his flank shall rest on Sidon.” (Genesis 49:13)
Rabbeinu Ovadiah Sforno comments that this refers to the tribe’s dealing in the international trade of commodities found in its marine and land portion, such as: the tarit [a small fish], the chilazon [the source of Tekhelet], and white glass. This coincides with the blessing given to the tribe of Zevulun by Moshe before his death:
“Rejoice, O Zevulun, on your journeys,
And Issachar, in your tents…
For they draw from the riches of the sea
And the hidden hoards of the sand.” (Deuteronomy 33:18-19)
The source of Sforno’s comment is in the Talmud, Megillah 6a:
Zevulun said before G-d, “Master of the Universe, to my brothers you gave fields and vineyards and to me you gave mountains and hills. To my brothers you gave land and to me you gave seas and rivers.” He answered, “Everybody needs you for the chilazon, as it says, ‘and the hidden hoards of the sand.'” Rav Yosef learns, “hoards” is the chilazon, “hidden” is the tarit, and “of the sand” is white glass.
The Radzyner Rebbe chose this verse as the title of the first of his trilogy on the topic of Tekhelet.