How to clean a 50 yr old oak hardwood floor?

In cleaning up my mother-in-laws house, we removed the carpet throughout the house to find really pretty oak hardwood floors, that once cleaned, look pretty good. However, in several of the bedrooms, the carpet pad that was used had black pieces of foam in it. This carpet pad has been under this old carpet about 30 yrs. For some reason, the these black spots bled into the oak flooring. I tried many things, short of sanding, to remove these spots, but did not have much luck. I did use the "Murphy's Soap" to clean the floor, and some of the spots did come off, but not all of them.

Thanks,
Richard

 

You will need to sand them if you want them clean.

 

The floor, other than these few black spots, is in perfect condition and does not need to be sanded and refinished.

Any other ideas other than sanding?

Thank you.

 

Area rugs.