How can you prevent black stains from the hardwood floors?

A lot of hardwoods bruise and there really isn't anything you can do to prevent it other than throwing down some rugs in areas where there is a lot of foot traffic or replacing it with a harder wood.

It's not enough to know that the floor is "pine" or "cherry" -- you have to know what kind of pine or cherry it is as the hardness can be very different. For example, American cherry has a Janka rating of 950, Brazilian cherry has a rating of 2350.

Watch out for water.

Almost every black stain I've ever had, other than heel marks, have come from the moisture on the bottom of potted plants.

Impossible to get out as well.

Joe(keep everything dry)Nation