Black garlic, a product obtained from fresh garlic (Allium sativum L.) under specific temperature and humidity conditions, has garnered significant scientific attention due to its enhanced organoleptic qualities and superior health-promoting properties compared to the fresh product. The Maillard reaction, alongside other chemical transformations, converts pungent organosulfur compounds like allicin into stable, highly bioactive compounds such as S-allyl-l-cysteine and S-allylmercaptocysteine, while also increasing the levels of polyphenols and other potent antioxidants. This review provides a comprehensive overview of black garlic, detailing the impact of its unique ripening process on its chemical composition, encompassing changes in sugars, proteins, amino acids, lipids, vitamins, minerals, and organic acids. We explore how variations in temperature and humidity during production influence the final product quality attributes and bioactive compound profiles. Furthermore, we extensively discuss the diverse biological activities of black garlic and its key constituents, including significant antioxidant, anti-inflammatory, anticancer, immunostimulatory, antiallergic, hepatoprotective, antidiabetic, and antiobesity effects. By synthesizing current research, this review highlights black garlic potential as a functional food and a source of therapeutic agents, while also emphasizing the need for standardized production methods to ensure consistent quality and maximize its health benefits.

Black garlic peels (BGPs) are a largely underutilized by-product despite representing an untapped source of bioactive compounds. This study presents a sustainable upcycling protocol for black garlic peels, evaluating natural deep eutectic solvents (NaDES) to develop multifunctional extracts for green cosmetics. Following the screening of four eutectic mixtures, the choline chloride–lactic acid (ChCl:LA) system demonstrated the highest extraction efficiency. The optimized extract yielded a remarkable total phenolic content (5216.61 µg GAE/mL) and strong antioxidant capacity, confirmed by DPPH and FRAP assays, associated with recovering both free and bound phenolic fractions. Subsequent HPLC profiling characterized the extract, and the comparative analysis explicitly demonstrated that antimicrobial activity is entirely driven by and identical to the pure eutectic solvent vehicle rather than the extracted garlic biomass, with broad-spectrum efficacy against C. albicans, P. aeruginosa, and S. aureus. To evaluate its cosmeceutical potential, the extract was incorporated into emulsions (5%, 10%, 15% w/w) with inorganic or organic UV filters. Although the direct Sun Protection Factor (SPF) and the UVA Protection Factor (UVA-PF) did not show enhancing results, a photochemiluminescence (PCL) analysis revealed a synergistic behaviour with organic filters, successfully boosting the formulation’s biological antioxidant shield. This pioneering work highlights BGP’s upcycling potential, proposing NaDES extracts as highly promising multifunctional, antioxidant, and antimicrobial ingredients for next-generation cosmeceuticals.