hello sidharth...Fluid is called newtonian if it obeys newton law of viscosity.
that is shear stress and strain rate exhibits linear relationship.
while blood does not obey this law and it is non-newtonian fluid. it is fall under the category of pseudo-plastic fluid. i am going to attach the graph between shear stress and strain rate of different types of fluids, you can see, in case of blood there is no linear relationship between shear stress and strain rate.
hope you understand.