Blackcurrant (Ribes nigrum L.), a nutrient-rich cold-climate berry, accumulates ascorbic acid (AsA) and flavonoids critical for fruit quality, yet their regulatory mechanisms during development remain poorly characterized. This study systematically investigated AsA and flavonoid dynamics across four developmental stages (young, expansion, veraison, ripe) in two contrasting varieties, 'Adelinia' and 'Heifeng', while integrating transcriptomics to elucidate metabolic pathways and regulatory networks. We observed a progressive decline in AsA content during fruit maturation, governed by coordinated regulation of biosynthesis (GDP-L-galactose phosphorylase-driven) and recycling pathways (mediated by monodehydroascorbate reductase). Flavonoid levels peaked at the young fruit stage, sharply decreased during veraison, and showed varietal specificity, with 'Heifeng' exhibiting higher accumulation. Co-expression networks identified 4 core structural genes and 6 transcription factors (TFs) regulating AsA metabolism, alongside 8 structural genes and 9 TFs associated with flavonoid biosynthesis. Comparative analysis of fruit size revealed divergent hormone signaling between varieties, with auxin- and cytokinin-related DEGs in the plant hormone transduction pathway (ko04075) strongly correlated with cell expansion. Photosynthesis-antenna protein pathway genes (ko00196) further contributed to size variation, suggesting energy allocation trade-offs during ripening. These findings advance the molecular understanding of AsA and flavonoid regulation in blackcurrant, highlighting cultivar-specific metabolic strategies. The identified genes and TFs provide actionable targets for breeding programs aimed at enhancing nutritional quality and yield, while insights into hormone signaling offer practical frameworks for optimizing growth regulator applications in cultivation.