Where does the orthodox tradition on wearing black long cassocks come from and what is their purpose? Here is a wonderful article that discusses this matter:
“People have sometimes approached me and asked why Orthodox priests (as well as deacons, monks and bishops) traditionally wear black cassocks. In the earliest Church, it does not seem that clergy wore any distinctive garb, except of course liturgical vestments, which in some cases also were worn outside the Divine services. For example, sometimes bishops and priests wore the phelonion like regular clothing, as did the deacons with the sticharion (in the West dalmatic).
It seems reasonable to assume this was at least partly due to the facts that 1) Christians were frequently persecuted during the first centuries, and self-identifying marks like clerical garb would therefore in some cases be tantamount to suicide; and 2) there was, at that time, no pressing need to alter their standard wear. The early priesthood was in many ways understood in the context of the Levitical priesthood inherited from the Jewish tradition by the early Church, and standard (non-liturgical) wear for them was not always identifiably different from that of others.
So for the first three centuries, priests wore more or less the same clothes as ordinary people. The tunics were normal clothes. But when it became fashionable to wear shorter robes, most priests kept the longer more conservative tunics, and thus they stood out from other people. As fashions changed in the Roman Empire, it seemed right for the ordained clergy not to follow the vain changes of worldly styles.
The Council of Braga in Portugal (572) seem to be one of the first synods to mandate that clergy wear a tunic reaching to the feet.
The Quinisext Council (692, i.e. the conclusion of the Sixth Ecumenical Council) stated: “None of those who are in the catalogue of the clergy shall wear clothes unsuited to them, either while still living in town or when on a journey: but they shall wear such clothes as are assigned to those who belong to the clergy.” (Canon XXVII)
Responding to reports of laxity in Britain, Pope John VIII (c. 875) admonished the Archbishops of Canterbury and York to ensure that their clergy wore proper attire, particularly long tunics.
The English word “cassock” derives from the early French casaque, meaning “a long coat.” The Russian word for the cassock is подрясник (podryasnik), and the Greek is αντερί (anteri) or ράσον (rason).
The color black indicates spiritual poverty – it is historically the easiest and cheapest color to dye fabric with. Moreover, black is a color of mourning and death for the priest, the symbolism is dying to oneself to rise and serve the Lord as well as giving witness of the Kingdom yet to come. Black is associated with sorrow but in the case of priestly robe this color has another symbolic meaning. A black cassock is to remind a priest that he ‘dies to the world’ every day and immerses in eternity. Blackness also symbolizes giving up bright colors and thus giving up what the world brings, its glittering, honors and entertainment. Also, as an Archpriest once pointed out to me, stains are readily visible on black, reminding the priest that he is held to a higher standard. His sins and failings will be more visible and judged harsher, than those of other people. In our very secular world, the wearing of the cassock continues to be a visible sign of belief and of the consecration of one’s life to the service of the Lord and His Church.”