How long should bruising last after having a blood test taken?

About one week. Bruises/hematomas are caused by a ruptture of underlying blood vesses with subsequent release of blood contents. This often happens during blood tests. It takes about 7 days to clear this from the tissue.
Several days. After blood is drawn, the puncture needs to heal, and spilled blood will be metabolized. The breakdown of blood goes through a defined pathway, creating the various colors (red, blue-purple, light green, light brown, dark brown) that we recognize through the skin as heme is broken down. It is variable for each person and so can take 4 - 20 days.