This is Mavra Volia beach, meaning "black pebbles".
It's probably easy to guess why the pebbles are black. Yes, you guessed it right, the soil is volcanic! This beach was created by volcanic activity, a long long time ago. The volcano is called Psaronas and don't you worry: it's a dormant one.
Access is easy through the road network. Note that there is little or no shade on the beach, and the pebbles can heat up quite a lot. So make sure you visit at times of the day when the sunlight isn't at its strongest. Also, there is usually a small kiosk at the beach in case you need a cold drink or a snack. As for tavernas, you'll find them at nearby Emborios, at a walking distance from Mavra Volia.