The blood weakening wards where formed around the horocrux cave by Lord Voldemort sometime between being taken to the seaside cave as a child (early to mid nineteen thirties) and regulus black's death/first fall by Harry Potter in 1979 and 1981 respectively. The blood "wards" - likely some form of jinx - are used to weaken a person physically by taking their blood which will likely help to finish the person off once they have drank the Drink of Despair and encountered a nasty army of inferi.