Rubus × 'Eldorado' is a historic, erect blackberry cultivar that emerged in the late 19th century, believed to have originated as a chance seedling of the wild highbush blackberry, 'Rubus allegheniensis' and the wild sawtooth blackberry, 'Rubus argutus'. While its exact parentage is a product of natural hybridization rather than laboratory breeding, it is widely considered a superior selection of the wild 'Allegheny' blackberry line crossed with local northern brambles. This lineage provided 'Eldorado' with its hallmark cold hardiness and the ability to thrive in the fluctuating climates of the Midwestern and Northeastern United States, where other early cultivars often failed. It served as a foundational genetic bridge between the rugged, seedy wild berries of North America and the refined, large-fruited cultivars of the modern era.

The fruit of 'Eldorado' is medium-to-large, jet-black, and renowned for its sweet, honey-like flavor and notably small seeds. Unlike many wild parents that possess a sharp acidity, this cultivar was selected for its high sweetness and melting texture, making it a favorite for "fresh-from-the-cane" consumption. Its robust resistance to orange rust—a common ailment of its parent species—made it an ethnobiological success story in early American pomology, as it allowed for the reliable cultivation of blackberries in home orchards without the constant threat of crop failure.