The black square denotes a "shielded" nuclide. That is, a nuclide that has a fission yield of greater than 2.5E-6% but that has a stable isobar one unit lower in Z, so that it (the shielded nuclide) is not produced as a daughter in the beta decay chain. See the Chart of Nuclides, 14th Ed., page 10, second column, penultimate paragraph. Charlie Willis