Catalyzes the reduction of nicotine blue to its hydroquinone form. Nicotine blue is the name given to the compound formed by the autocatalytic condensation of two molecules of 2,3,6-trihydroxypyridine, an intermediate in the nicotine degradation pathway. May play a role in preventing the intracellular formation of nicotine blue semiquinone radicals, which by redox cycling would lead to the formation of toxic reactive oxygen species. Besides nicotine blue, several other quinones are reduced by nboR. 3,3'-bipyridine-2,2',5,5',6,6'-hexol + NADP(+) = (E)-2,2',5,5'-tetrahydroxy-6H,6'H-(3,3'-bipyridinylidene)-6,6'-dione + 3 H(+) + NADPH 3,3'-bipyridine-2,2',5,5',6,6'-hexol + NAD(+) = (E)-2,2',5,5'-tetrahydroxy-6H,6'H-(3,3'-bipyridinylidene)-6,6'-dione + 3 H(+) + NADH Alkaloid degradation; nicotine degradation. Homotetramer.