Dear Walter
The dark mark, if you are lucky is just a difference in the aging of the finish. The rug has simply slowed the ambering of the finish (if it's poly, you didn't say), and once removed for about 6 month it should fade.
But it may be a more sinister problem. There may be a synthetic rubber backing on the carpet, or under-pad. If these were made with cheap plasticizers they may have migrated into the floor finish (which is a kind of plastic also). The only cure for this is to remove the top layer of finish affected. You may be able to take a non-abrasive white nylon pad, and a PH neutral cleaner (as mentioned in my article available in the search box at the top of this web page) and scrub some of it off. If not use 120 grit sandpaper to gently sand the surface off, until the dark line is gone. Then of course you will need to re-coat the entire kitchen floor. This you should be doing every 2-4 years anyway.
So, after trying 6 months of the first method, and also trying the gentle white pad scrubbing, you may just have to sand off the spot, like I suggested, and then re-coat the whole kitchen floor. Use the same floor finish as before, I hope you know the brand.
Oh, and don't put back that rub. I use only pure wool rugs on my floor, with no under-pads.
