Basalt: The ideal black building block (Pictures Included ...
Nice idea! Mind if I make a suggestion? Continental crust is primarily granite and oceanic crust is primarily basalt (as you mentioned). If this block were to be added, it could replace nearly all of the stone layers in ocean biomes instead of just being where lava is. This gives another reason to go under the waves and makes basalt a special, not as easy to get building material.