Dark hardwood floor stain
Need more photos- but do you mean on the right? If so that looks to be more junk on top of your floor.

You may have to, depending on how bad they are, lever them out and flip them upside down. I had to rotate and flip about 30 boards in my house, and they came out just fine. I ended up getting some red oak stock as well to make the few I couldn't fix, but put them out of the way in corners. 

Follow the guides for sanding and it should come up. However you may still have to flip them if they don't look right. Take for instance the nails- they've wrecked the wood around them and you won't bleach them out with Oxalate easily. Personally I wouldn't judge that harshly if I saw nails like that because I believe it adds character. And using oxalate is nasty.

Once everything was drum sanded down (be careful!), stained, and sealed, the place looked gorgeous.

-at least until a pipe burst and flooded everything in the middle of winter, gutting the place.