Brassica nigra (black mustard) is one of the three diploid Brassica species of U’s triangle (U, 1935) and contributed a genome to the formation of the amphidiploid species B. juncea and B. carinata. Although no longer grown extensively the species could serve as an important repository for untapped variation that can be exploited in Brassica breeding. Our group has generated three reference assemblies for two B. nigra genotypes (Ni100 and CN115125). Both genotypes were sequenced using long-read sequencing (Oxford Nanopore Technology), and a de novo short read assembly was also generated for Ni100. These three assemblies have been used to validate the use of ONT sequencing for polyploid genome assembly and methylation analyses. In addition, a comprehensive gene annotation and repeat element analysis has been carried out and is described in the following manuscript: