Chlorine bleach kills the bacteria associated with pet urine, as do other disinfectants. Unfortunately, sometimes the urine and its associated bacteria make it to the area beneath the hardwood ? the subfloor. When this happens it may require removal of the hardwood to get to the bacteria with a disinfectant. If the hardwood flooring is tongue and groove the help of a flooring contractor may be in order. However, if the flooring is square edged (the most common type in older homes) then it is probably a job that you can do yourself. Remember, sanding out the black stains won't get rid of the smell if the urine made it to the area beneath the hardwood.