Possibly a piece of anthracite coal. It should scratch fairly easy with a knife tip if it is. Anthracite, and other coals, were common fuel materials for old ships and are often found on beaches. A good specimen if that's what it turns up to be.
You won't find obsidian or black flint in MA beaches, no obsidian here on the east coast. Anthracite coal also has conchoidal fracturing and is a fairly common beach find here in the east. Like DD says, a simple hardness test with a knife will tell the tale.