There is nothing like good or bad blood in the human body. There are diseases or conditions where some biochemical component/drug/protein etc. may increase in the blood, and in those conditions blood needs to be filtered (dialysis, plasmapheresis) or exchanged (as in some diseases in newborns).
Leeches are flattened, segmented water worms which have two suckers, one at each end of their bodies. Most are parasites i.e. they attach themselves to other animals and live by sucking bodily fluids, usually blood, from their host. Most have well developed jaws with gristly mouth parts to break through the skin of their victims and suck fluids. Their saliva contains a pain killer so the victim does not feel them break the skin. The saliva also contains an anticoagulant (a chemical that stops the blood from clotting) called hirudin.
The blood sucking leech is used in modern medical surgery. The leech removes excess blood which can cause swelling and pain. It gives the body time to regrow the small capillaries needed to allow the proper circulation of blood and this is generally made use of on replanted digits, ring avulsion injuries and in small free flaps where there is good arterial inflow but no venous outflow. The leech's main therapeutic benefits are not derived from the blood removed during biting (although this may provide dramatic relief at first), but from the anticoagulant and vasodilator contained in the leech saliva which permits the wound to ooze up to 50 mls of blood for up to 48 hours. The surgical aim then is to produce an adequate venous outflow from the tissue by adjusting the number of leech applications and thereby bite wounds to suit the clinical applications until new vessel ingrowth around flap margins develops sufficiently to restore effective venous drainage.
The anticoagulant Hirudin present in leech saliva is being used as a medicine to prevent clotting of blood in conditions like heart attack, strokes and venous thrombosis.