Hence a decreasing number of rabbis and chazanim wear the caps and gowns that once were almost ubiquitous, though the argument of chukkat ha-goy sits strangely because special robes and headcovering have highly respectable Jewish origins in the official garb of the ancient kohanim.
It is not whether robes should be worn that we will address today, but the question of their colour. Black is probably too sombre, though it has the advantage of not showing the dirt – and the sages said that a talmid chacham must not have a stain or speck of dirt on his clothing (Shab. 114a; Maimonides, Hilchot De’ot chapter 5). But the Biblical precedent makes it clear that the robe was “all made of blue” (Ex. 39:22).
The blue cannot have been chosen at random, since every Israelite had a thread of blue in the fringes on his garments (Num. 15:38). Blue resembled the sea, the sea resembled the heavens and the heavens resembled the Divine Throne (Sotah 17a). Not only the priest with a robe of blue was answerable to God, but every Israelite with a blue thread in his tzitzit.
To be authentic, rabbinical robes, where they exist, ought to be blue and not black. Indeed in Talmudic sources black garments are not a good sign. On Yom Kippur, Shimon HaTzaddik found that an old man in white entered and left the Holy of Holies with him, but one year the old man was in black and did not leave the Holy of Holies with him; within a short time Shimon died (Yoma 39b).
A kohen whom the Sanhedrin deemed unfit had to put on black clothing and go away (Yoma 19a). Black shoes or at least black laces were a sign of mourning (Bava Kamma 59a/b).
Hence, if robes in themselves are not chukkat ha-goy, the choice of black material might be.