Why would it be appropriate to use a word in the figurative sense only if it is used in singular form, but if used in plural form, it's inappropriate?
who said inapriopriate in plural - Poles say often about themselves : 'Białych murzynów z nas zrobili' - They made white slaves of us. (slave is just the most appriopriate translation into English and not a literal one)
No it's not.Murzyn or murzyni may sound a bit offensive to Africans when used in Sto lat za murzynami indicating low (lowest) level of development or zapierdalam jak biały murzyn as clearly nobody wants to be biały murzyn.
but it doesn't make a word murzyn dismissive even not to say derogatory - it just comes from the observation black Africans were backward
the point of the criticism in the word is towards those who use those murzyni
This is the point really. When I used to do factory work back in the eighties, the crappest job in the factory was always called the 'Irish job'. A similarity in many ways.
one big difference is there were no actual 'murzyns' black people doing those 'Irish jobs' in Poland- and in Polish also it bears a connotation that ;murzyn is a over-exploited (slaved) with no hint to their inferiorness or lower capabilities - as I said figurative meaning of 'murzyn' has a definite point of criticism toward those who are using the 'murzyn's' work