It's called bleeding obsidian...
Some information on it...
Nothing is known about the new block - and official sightings are scarce and unproven. It's existence is confirmed only by - and we checked - the inclusion of the texture for this block in the source code. Some speculate that this is lapis lazuli mixed in with obsidian, and others speculate that it is a new form of material, an Obsidian-Diamond hybrid to make the ultimate tools. However, so far nobody knows what's going on, and even the name isn't official. We'll keep you posted on updates as they happen, as it's possible this block won't have a purpose until a future update - but looking at what the block looks like, it's likely going to have a solid impact.
Source: http://gamerant.com/minecraft-13-update ... hnj-69219/
