Sjogrens can be very problematic for the oral cavity. With a reduced salivary flow many complications can arise--the number one being an increase in plaque which increases the risk of cavities and perio disease.
To help reduce these risks :1. Have exceptional homecare--brushing with an electric toothbrush, flossing daily. 2. Prescription Home Fluoride Daily (Gel Kam is good) or you can get a Rx from your dentist/doctor. 3. Artificial Salivary Substitute such as Biotene as needed. 4. Keep mouth hydrated by drinking lots of water and using sugar free mints 5. Diet control--limit sweets, carbohydrates especially ones that are sticky and difficult to clear 6. Regular dental appointments
About the possible decay around the crown--the dark could be the metal underneath but I would pop into the dentist to have him take a look