Our previous studies have demonstrated that the statistical distributions of ultrasonic backscattered signals were able to characterize hemodynamic properties of flowing blood associated with red cell aggregation and coagulation. Yet very little is still known about the phenomenon of spatial variation of "black hole" (BH) in the laminar flow with signal statistics of the flowing blood. To further explore this issue, measurements were performed from the porcine blood (with hematocrits of 20 and 50%) circulating in a mock flow loop under various steady laminar flows at mean flow velocities ranged from 1.5 to 12.2 cm/s using a 10 MHz focused ultrasonic transducer. Results showed that the BH phenomenon was apparent for the 50% blood flowing at a low velocity. The BH phenomenon tended to be decreased with the increase of flow velocity and was hardly observed in the 20% blood. The probability density function (PDF) of signals backscattered from flowing blood tended to distribute as pre-Rayleigh statistics with the corresponding mean Nakagami parameter is less than 1. The scaling parameters and Nakagami parameters are respectively decreasing and increasing respectively to the increase of flow velocity. Moreover, larger scaling parameters and smaller Nakagami parameters were obtained respectively from 50% and 20% bloods. The spatial distribution of red cell aggregation and shear rate in the flow tube is a predominant factor leading to the statistical variations of ultrasonic backscattering in the flowing blood and that the formation of BH phenomenon tended to decrease the Nakagami parameter.