A few months ago I started making my own laundry soap. It is a mix of Borax and Washing Soda. It is really cheap, you don't have to use very much and I can use it on my cloth diapers if I'm out of other soap.
I read the back of the boxes when I got them but didn't really think much of it at the time. They list all the things you can do with each of them.
There are a few oil like spots on my carpet right at the entry way of the living and every time I walk by them I think "I wonder if that washing soda would get that out? The box says it will.". Well, I can now confirm it will! No more black spots!!!! Just mix with hot water and scrub away. I patted it with a dry cloth in between scrubbing to absorb the water. I wish I had before and after pics.
The only problem is while I was on the floor crawling around I kept finding other spots to scrub! I got out another black spot in front of the TV and a grape jelly stain that has been there forever. The person who dropped grape jelly will not be named to protect our marriage:)
UPDATE: It does tend to get your carpet so clean that it might leave it a little lighter then the carpet around it. It's not really noticeable unless you are looking at it from an angle. It doesn't bother me near as much as the black spots did.