In April 1960, 70 western-oriented countries issued stamps for the proclaimed UN World Refugee Year. Since most of the refugees at that time came from communist countries, four of them protested by declaring these stamps invalid. These countries were Poland, Czechoslavakia, Sowiet Union and Romania. Some covers were normally delivered without objection, but most were not delivered. Poland blackened the stamps occasionally, but mostly removed them.
From the Netherlands only one cover is known. This registered letter was sent from the Netherlands to Poland on 19/4/1960. Only the postage stamps “World Refugee Year” were blackened. The 1 cent postage stamp is not. So the blackening was clearly only against the World Refugee postage stamp.
More information on postal war, see www.postalwar.info.