Fresh is best when it comes to blood.... The first drink after a kill is always the most revitalizing....
Taste can vary greatly depending on the quality of the animal... great tasting blood is a very good indicator of a healthy animal....Some of the animals that are not so good, often have a sour component, and I have even noticed a slight burning in the back of my throat followed by a bad aftertaste when drinking subprime blood. Quality blood on the other hand taste a lot like clean ocean water.
I will get a little under a gallon a month from the sheep I butcher and will drink about half during the first week, keeping it refrigerated.... and freeze the other half of it in halfpint glass jars, to be thawed out at a later date.
I've noticed a change in the taste and quality of blood within a few days after slaughter, and don't like drinking anything after about a week.....even the thawed blood isn't as good....There must be vital living factors and enzymes within living blood that diminishes soon after the death of the animal.....This could explain why vampires where always portrayed drinking blood directly from the source.