How did the concept of 'uniform ' [ as in dress] come into place. ? i think it must have been [ in the early days , to identify people of a clan/race. Imagine a battle where you do not know your foe or friend. so uniform originated there. So that people identified who is on our side and who is not.
In the early days synthetic colours were not available and mostly natural dyes were used. As purple was rare and costly it was only used by the royalty in the roman empires. We read about the ships coming all the way to india for indigo.
Thus certain colours came to be identified by certain professions, mainly because of the purchasing power.
When all the world was busy being colourful , one colour - black stood out and said , i am not touched by this display of colours. I am nuetral and above all this things.
Law is not a flamboyaunt display of our attire but of our knowledge. Hence black may have been chosen.
I may be wrong but shree has rightly pointed out the historical significance.