Saying that something has a density of 3.14159265359 ug / dL is useful when we are talking about molecules in solutions and understanding chemical reactions.
Notice that we are measuring pi for mass (in micrograms) and deciliters for volume. Since pi is an irrational number (something that is not well defined and depends on the scale involved ie how many decimal places you want to go before deciding to round the value of pi) the actual value of density depends on the scale used to measure it.
Because of the scale that we use to measure things in physiology is often different from the scale of atoms and chemistry we need to use specific gravity.
Specific gravity is a DIMENSIONLESS quantity that measures how close the density of your solution is to water (closer to 1 = closer to water). This is useful because dimensionless quantities are scale invariant. Since water is the universal solvent, specific gravity is a pragmatic measurement of how dense a solution is RELATIVE to that of water.
Viscosity measures how much resistance their is to flow.
DENSITY HAS VERY LITTLE TO DO WITH VISCOSITY!!!
Viscosity of a solution (blood in this example) in physiology is mostly dependent on the underlying chemistry (intermolecular forces), since temperature is kept mostly constant.
For example if you have higher than normal red blood cells, plateles, or inflammatory cytokines in your body you will have an increase in the blood viscosity via increasing the intermolecular forces at play in the solution ( proteins in the blood).