Edema legs feet ankles black/ blue sudden onset?

Fluid retention. Usually comes from one of 4 areas: heart, lungs, liver or kidney. The fluid shows up in the feet due to gravity. The black & blue discoloration may or may not be related to the swelling. Sometimes, the swelling can actually break a few tiny blood vessels to cause the black & blue, but it could be unrelated also. You need to see your doctor asap. Sudden onset of swelling can be serious.