The origin of a word odd irrelevant. The use of a word is all that matters.
the thing is it is a regular word for a black person of African origin (other black people like Australian aborigenes or Melanesians or even very dark south Indians have their own specific names and are not typically called 'Murzyni' though it was a bit different historically)
if someone wants to call a black person offensively in Polish they use words like 'czarnuch' (extremely offensive - surprisingly enough 'czernoch' is a regular word for black person in Czech language), 'bambus' (literally 'bamboo' but it originates from 'Murzynek Bambo' by Julian Tuwim) or 'asfalt' (literally tarmac)
other neutral words for black people have typically different meaning to murzyn: czarnoskóry means literally black-skinned (but remember that Murzyn has a meaning of African black) - so Australian aborigene is equally czarnoskóry, czarny - is a touch too close to czarnuch to be an innocent word; Afrykanin doesn't equal to murzyn as there are Arabic, Berberic, and Chamitic Africans who are not Murzyni (and there are also white Africans after all - biali Afrykanie)
Any word is racist if it is used in an aggressive or derogatory way.
murzyn doesn't carry offensive load because it is a regular word for a black African person - it's like stating that Englishman or Pole is derogatory - if Polish person wants to be derogatory towards the English they say 'Angole' not 'Anglicy', likewise if English people want to use derogatory, dismissive term for Polish people they say 'Pollacks' instead of 'Poles'