Not sure about the black stuff. Do the bees look and act healthy? A healthy hive will do house cleaning is why I ask.
Are the eggs in the bottom of the cells, like all the way? Queens will lay more than one egg per cell sometimes, in the bottom of the cells. Laying workers will lay multiple eggs, but can't quite reach the bottom to deposit the eggs.
Just keep an eye on them, and look to see that you have a good pattern of worker brood from your queen. Don't jump the gun on the laying worker idea, everything is prob just fine.
...JP