Walter C. Kaiser, Jr., has an interesting insight on the requirement that sacrifices be "without blemish": "The first principle is that happy communion with God rests on the willing consecration of the best fruit of our lives to God. Just as the peace offering, the sin offering, and the guilt offering required that all the fat be laid on the altar in dedication to God, so we are reminded that we owe God our best." (New Interpreter's Bible Commentary, vol. 1, p.563)
Symbolically that which is without blemish is the best of all possible choices. In our activities as human beings we have many choices - it behooves us to be on our best behavior at all times, because our very lives and the ability to make those choices are a gift from God.