>This usually occurs in the aftermath of a period of reduced blood flow to an area of the body. When circulation is fully restored following constriction of blood flow the blood vessels dilate and can become engorged with blood. This can cause discoloration ranging from dark red to the bruised appearance you described as well as exquisite pain.
>If our fin straps are too tight or we have forced our foot into the foot pocket too firmly we can create a steady mechanical pressure on the blood vessels. This is colloquially known as ‘fin foot’. Colder water can also contribute to the constriction of the blood vessels adding to the reduced blood flow to the area. Usually this condition does not require medical intervention other than pain management. Typically staying off the affected foot as much as possible, elevating the foot and applying warm compresses will help keep the affected area comfortable.
>Working with your local dive professional to confirm proper fit of your equipment may be in order as well.