Dog has a blue bump & licks stomach: infection or kidney issues?

There is not a photo attached to the post and that may help to give better advice. There are many reasons why Sassy may be licking her stomach and it includes allergies to fleas, allergies to something in the environment, food allergy or endocrine disease. Each one of these problems is detected and addressed differently and I recommend to discuss with your veterinarian which one to start hunting down first. Parasites are often ruled out with an oral medication like Simparica. Food allergy is best ruled out with a diet trial using a hydrolyzed protein. Endocrine diseases are detected with blood work. Environmental allergies are a little more difficult to detect but a response to therapy or a blood test can give an indication. Once you figure out and address the underlying cause, the licking should not come back.
There are a couple things you can do at home and they can help but they may not address the underlying cause. You can try adding in an omega 3 fatty acid supplement for dogs, this can help improve the skin health. You may also try changing her diet to something like Royal Canin or Nutro as they are aimed at skin health. Also protect the area from trauma by having Sassy wear an elizabethan collar at all times.
Renal disease can present like this but without bloodwork, it's difficult to know if renal disease is present or not. You are right that the bump could get infected. Again, you need a veterinarian for antibiotics. Here are two links which may be helpful for your research:
https://www.petcoach.co/article/kidney-disease-causes-signs-diagnosis-and-treatment/

https://www.petcoach.co/article/lumps-and-bumps-on-your-pet/

I think your best solution is to have Sassy examined. Some basic testing will answer a lot of your questions and calm your worries.

I hope this helps and please feel free to post any additional questions.