The observation of a hypoechoic zone around the center of large tubes (the "black hole" phenomenon) in ultrasound backscattering measurements with red blood cell (RBC) aggregation was reported for the first time in 1989. Since then, a very limited number of studies tried to explain its complex mechanisms. In this study, blood models characterized by different RBC aggregation levels were prepared by diluting horse blood plasma with a saline solution in different proportions. A laser reflectometry technique was used to characterize the RBC aggregation kinetics and cohesion forces between RBCs for each blood sample. The blood was circulated in a 12.7 mm diameter vertical tube. For each experimental flow condition tested, 25 or 15 power Doppler ultrasound measurements were performed across the tube with a 10-MHz system and insonation angles varying between 40 degrees to 70 degrees. For flow rates varying between 100 and 1250 mL/min, the "black hole" was observed in most measurements performed with different aggregating RBC models. The "black hole" was more pronounced for RBCs with a high kinetics of aggregation and measurements with increasing Doppler angles. Previous studies suggested that this phenomenon is due to tube entrance effects, and the reduction of RBC aggregation at very low shear rates around the center of the tube. In the present study, the "black hole" was observed for shear rates up to 25 s(-1). It is suggested that the structural organization and orientation of RBC rouleaux may participate in the mechanism leading to the "black hole" phenomenon. A schematic representation of the rheological behavior of horse RBCs in a large tube under steady flow is presented.