In conventional dual-mode ultrasound imaging systems, B-mode and Doppler mode typically operate in an alternating transmission manner to achieve structural imaging and blood flow velocity estimation. However, this time-division multiplexing strategy significantly limits the Doppler sampling rate and temporal resolution, resulting in discontinuous hemodynamic signals and increased complexity in hardware scheduling and real-time processing. To overcome this limitation and realize waveform integration, this paper proposes a blood flow velocity estimation method based on Differential Frequency (DF) processing. The proposed approach exploits high-frequency broadband transmission signals and performs frequency-difference operations at the receiver to extract the frequency difference between two high-frequency components, thereby generating an equivalent low-frequency signal for velocity estimation without alternating transmissions. This DF strategy maintains high-resolution characteristics while substantially reducing signal processing complexity and hardware bandwidth requirements, achieving waveform resource sharing between structural imaging and flow detection. In addition, a covariance-matrix-averaging-based robust processing strategy is introduced to suppress artifacts arising from differential operations, improving estimation stability and accuracy. This research mainly focuses on reusing high-frequency broadband B-mode transmission resources for blood flow velocity estimation, aiming to unify imaging modes and enhance real-time performance. Theoretical analysis and simulation results demonstrate that the proposed method maintains high estimation accuracy and temporal continuity even under low SNR conditions. Compared with conventional low-frequency and sparse estimation methods, it exhibits lower computational complexity and stronger artifact suppression. In summary, the proposed differential-frequency-based blood-flow velocity estimation method offers a feasible and efficient signal processing approach for unified ultrasound waveform design. It maintains imaging quality and estimation accuracy while significantly improving system efficiency, making it particularly suitable for future miniaturized and portable applications, such as wearable home-use ultrasound devices and handheld blood-flow monitors that support long-term real-time monitoring. This work establishes a solid technical foundation for high-frame-rate, real-time, and miniaturized ultrasound imaging systems.