It's hard to tell, but it looks like the finish has worn away. If that's the case, the only way to fix it is to have them sanded and refinished.

The finish has worn off and the 'black' is from dirt hitting the wood (not the plastic finish on top). The dirt has iron deposits in it. When the dirt gets wet (cleaning, snow, rain, anything that causes the dirt to get wet) the iron deposits react with the wood, creating a black spot.

The 'black' iron spots is a BIG indication the finish needs to be sanded down and the wood refinished. To achieve a full sand/refinish you need either solid hardwood OR an engineered hardwood with 3mm+ wear layer.

If you can refinish these, you will want to do it sooner (much sooner) than later. I'm seeing black leaching into the wood on the short edges...and many of the long edges. The longer you leave this the harder it will be to sand off the 'black spots'. I would plan to do this sometime this year.