Horneblende is a dark green and sometimes looks like it is black. It is usually in a rectangular shape or can have thin crystals in the stone. Horneblende is usually found in metamorphic rock and sometimes in igneous rock. That is my best guess without being able to see it.
I have some Epidote in my collection but it is usually a medium green. If your rock is wet or has moss on it you could try drying/cleaning it to get a better idea of the color. Epidote is pretty common to find so it may be a possibility.
I would need more information to know exactly what kind of rock you have. Is it a rock that looks more solid based or is it glassy? Diopside and Jadeite would qualify at times as a darker green but also have lighter greens available. Enstatite is also green in color.