essentially a blemish is an active inflammation of some type, and a red mark is the aftermath, the infection is gone, it can be flat but tends to leave a red mark....that is called hyperpigmentation....it fades, faster for some than for others and of course it depends on the severity of the blemish

but if the skin is flat and just discolored, you are lucky that will fade with time...you can speed it along with peels and AHAs...if it's an indent or ice pick then the battle to eradicate it is more difficult

hope that helps