Actually, more successful example was North Carolina where black-white alliance that was Populist-Republican (Populist-Democratic alliances were possible in mid-west and west but not in south) gained control of state government in 1890s. Unfortunately, this victory was both pyrrhic and singular, setting off terror of looting, burning, and lynching called 'white supremacy jubilees' leading to restoration of racist rule.
Black-white farmer alliances (championed by Georgia's Tom Watson who would in later bitter and broken days adopt virulent racist attitudes) against monied interests seemed possible in late 1880s. Southern Alliance had over 3 million members and Colored Farmers Alliance had about 1.25 million. Of course, fact that white Alliance excluded black farmers from their organization indicated already-present limitations. Moreover, willingess of black populists to work with white populists to further common interests was, more often than not, scuttled by latter, most of whom were landowners concerned with changing credit system. Black populists, most of whom were laborers (many came out Knights of Labor), experienced intimidation and violence at the hands of white populists when their political and economic demands were seen as straying too far from interests of rural white communities.
White populist commitment to working with blacks was too often limited to seeking black votes for People's Party candidates. The party, for example, never took up anti-lynching legislation, addressed bans on black jury duty, or considered unequal funding of black education (rather, black proposals on these and related matters fueled white hostility). In fact, sizeable contingent of white People's Party members supported disenfranchising blacks. Following 1896 disaster, when party destroyed itself on petard of Bryan's Democratic candidacy (his running mate was conservative railroad tycoon Arthur Sewall, his populist rhetoric was doublespeak except on free silver issue, he never campaigned as Populist despite party's nomination), white southern populists adopted, wholesale, racist 'white supremacy' position and generally scapegoated blacks for failures of populist movement.
Goodwyn's book is good read and important contribution but it suffers from romanticism charactering good bit of late New Left historiography. Michael Hoover