This is most likely an over the counter sleep aid which contains Diphenhydramine 25 mg (NDC 76168-015). If you check the page on DailyMed and go to the section called "Ingredients and Appearance", it describes a blue oblong tablet (13mm) with no score line and imprint code DB. The fact that it's an OTC product could also help to explain why it's been so hard to identify. They also have images of product packaging on that page, and on the front of the package is a small image of a blue tablet. Feel free to have a look and let me know if that's what you have?