What is the dark blue diamond-shaped pill with 100 on one side?

The dark blue, diamond-shaped pill with "100" on one side is identified as Diphenhydramine hydrochloride 100 mg, an antihistamine commonly used to relieve symptoms of Allergies, hay fever, and the common cold. It can also be used as a sleep aid due to its sedative properties. Always consult a healthcare professional for proper usage and potential side effects.