Achieving a clear vision through turbid fluids is a highly desirable goal in microfluidics. In particular, observing particles dipped inside blood shows fascinating perspectives in all fields of biomedical research. White-light microscopy cannot provide clear imaging due to the strong scattering of light by red blood cells. Here we solve the problem by Digital Holography microscopy. We show that, in cases where the blood flows along a microfluidic channel at sufficient speed, the hologram acts as a selective filter. This occurs due to the Doppler frequency shift experienced by the photons hitting the red blood cells, discarding the unwanted scattering. In cases where the blood flow is not quick enough to take advantage of the Doppler shift, multiple holograms can be processed to produce a clear image of the object. We show that the correlation coefficients between multiple acquisitions at different fluid speeds can be adopted to study the visibility of the fringes due to the moving colloidal particles in the medium. Hence, we estimate the threshold velocity required to completely discard all the scattered photons. In this way the object is seen as dipped in a transparent liquid, thus completely eliminating the negative effect of turbidity on the imaging.