<p>This study investigated the effects of orange juice (OJ) addition and amino acids with different chemical structures on anthocyanin stability in black mulberry juice (BMJ), and to identify their roles in copigmentation. BMJ (0.65% acidity) was blended with OJ to increase acidity (1.34%). Five amino acids (aspartic acid (Asp), phenylalanine (Phe), valine (Val), asparagine (Asn), and lysine (Lys)) were added to the BMJ: OJ system. Changes in total monomeric anthocyanins (TMA), colour density, polymeric colour ratio, anthocyanin profile, amino acid profile, and ascorbic acid contend were evaluated. All amino acids reduced TMAs (up to 5.4%). Asp promoted copigmentation by increasing A_max and colour density, while Phe, Val, and Asn induced condensation, and LYS accelerated anthocyanin degradation. Asp, Phe, and Asn reduced browning and polymeric colour formation through amino acid: o-quinone interactions or PPO inhibition via amino acid: Cu(II) chelation. In contrast, Lys underwent Maillard reaction, increasing browning and polymeric colour ratio. Asp, Phe, Val, Asn, and Lys formed chelates with Cu(II) and Fe(III) ions, increasing ascorbic acid stability. In contrast, Val combined with phenolic compounds reduced the protective effect on ascorbic acid. As a result of these reactions, the amino acid-added samples had lower free amino acid content than added (10.3–93.5%). The data suggest that copigmentation occurs only in Asp with an acidic side chain, suggesting van der Waals forces or electrostatic interactions between negatively charged Asp and positively charged anthocyanins in BMJ and BMJ: OJ media. Therefore, amino acids with acidic side chains are recommended as copigments.</p>