Ancient Russian Boyar Court Dress

I have seen pictures of the boyars sitting along each side of a hall before the tsar’s throne and they have on the same tall black hat and black robes. what is the origin if this odd outfit. 2) why were they dressed the same when they were
probably at different levels of greatness and different parts of Russia? 3)What were the boyars anyway? Nobles? Special nobles? Large landowners? My favorite boyar is Effrosinya in Eisenstein’s IVAN THE TERRIBLE movie, the one who peers from behind columns.