According to Joan, one of our most knowledgeable pet experts, the black things are mosquito larvae.
Clean your dog's water bowl every day using a solution of 1 part of Clorox bleach to 4 parts of water and add a few drops of dish detergent then rinse it thoroughly with fresh water.
Fill your dog's clean water bowl with fresh clean water a couple of times during the daytime.
Place it in an area where she can get to it easily but where bugs and debris cannot fall into it.
Mix a couple tablespoonfuls of applecider vinegar in her drinking water. It will repel mosquitoes and it won't hurt your dog to drink it.
Keep pets indoors at dawn, dusk, and early evening when mosquitoes are most active.
Eliminate areas of standing-water that can serve as breeding grounds for mosquitoes, and remember to recheck after each rainfall.
Here are photos of mosquito larvae. Did the things look like this?