We get bruises when muscle fibres and connective tissue are damaged but the skin above them doesn’t break. So when we bang into things or get hit by blunt objects the muscles are damaged and the small capillary blood vessels within the muscles break. The skin isn’t broken so the released blood has nowhere to go except to pool together and form a dark purplish bruise. Bruises change colour as they heal because the blood is broken down and re-absorbed back into the blood stream while the connective tissue is also repaired.
I sympathise with your recent bruise; I bruise like a peach!