1. A blackening processing method for a radiation image, in which a blackening process is performed on a predetermined subregion in a radiation image, the method comprising the steps of: i) partitioning the radiation image into a plurality of subregions, ii) setting a certain subregion, which is among the plurality of the subregions, as a blackening region, which is to be subjected to the blackening process, or as anon-blackening region, which is not to be subjected to the blackening process, iii) setting subregions, which are adjacent to each other among the plurality of the subregions, alternately as a blackening region and a non-blackening region by taking the certain subregion as a reference subregion, and iv) performing the blackening process on the subregion, which has thus been set as the blackening region.