This invention relates to a new and distinct variety of blackberry plant named ‘HJ-6’. The new variety resulted from seed produced by a hand-pollinated cross of two non-patented varieties: “Obsidian” as the female, and “Eaton” as the male. The new, trailing cultivar can be distinguished by its consistent, large fruit size and appearance during the late spring and summer period for the fresh market. The fruit of this cultivar is produced on dark red, strong, thorny canes. When plants are grown and pruned correctly, yield in central California, USA can be up to 9,000 crates/acres, and the fruit ships well.