In this study, we developed a laboratory scale mesocosm to study the conditions that favour the formation of black water blooms. Using water samples and sediment from an eutrophic lake, we analyzed the daily transformation of physical, biological and chemical conditions over several weeks. The results showed that a transformation of oxic-hypoxic-anoxic conditions characterized the formation of the black bloom. In anaerobic conditions, chromophoric dissolved organic matter (CDOM) increased due to the production of degradation products from decaying algae. CDOM reached a maximum on day 6 with the value of 4.48 m^-1. The blackness (FeS concentration) increased initially, then decreased to a constant concentration of 0.35mmol/L. CDOM values were significantly correlated with the blackness of water color. Water-leaving radiance was simulated under different CDOM conditions by Hydrolight and transposed using the CIE color matching functions. This showed a transition from green to brown which closely followed the increase in CDOM absorption. Results above clearly show that CDOM is a major factor leading to the formation of black blooms, and can be used to monitor their dynamics over time.