Blood is considered a suspension because it contains particles that can settle out.
A (heterogeneous) suspension is defined as a mixture between a liquid and particles of a solid. In the case, the particles do not dissolve. The particles and the liquid are mixed up so that the particles are dispersed throughout the liquid. They are "suspended" in the liquid. A key characteristic of a suspension is that the solid particles will settle and separate over time if left alone.
This is exactly what happens when anti-coagulated blood is left to stand alone for some time. Blood cells separate and settle from the plasma, the watery part of the blood.
