Blackout (BO) in breath-hold diving is attributed to cerebral hypoxia, yet direct observations are rare. We continuously recorded arterial oxygen saturation (SpO₂) and heart rate (HR) in 11 trained freedivers (5 females) performing two dynamic apneas (75 m, 100 m) using a waterproof forehead oximeter. One diver experienced BO at the end of a 100 m dive (SpO₂ 51 %), recovering within 5 s. Group SpO₂ fell from 98 ± 1 % to 77 ± 9 % (75 m) and 68 ± 9 % (100 m; range 51–83 %), while mean HR declined from 83 ± 12 to 43 ± 8 and 40 ± 4bpm, respectively. No arrhythmias were detected. Within-diver SpO₂ nadirs were consistent between distances (r = 0.93), whereas HR nadirs were not (r = 0.40). This case confirms BO can occur at SpO₂ values around 50 %, even in the absence of arrhythmia. The BO diver consistently showed the lowest SpO₂, indicating profound hypoxemia as the most likely contributing factor. Findings support individualized risk screening based on early desaturation patterns in submaximal dives.