The Dark Matter Block is stronger than Obsidian, making it completely indestructible, requiring a Dark Matter Pickaxe or better to mine it within a reasonable amount of time. This block's blast resistance, like the Red Matter Block, is equal to Bedrock. This means it cannot be destroyed by any explosion. However, while indestructible, it will not provide protection against Nukes or Nuclear Reactor explosions, as the blast passes through the block.
Please note that the Dark Matter Block cannot be crafted back into Dark Matter after being crafted. The only way of reverting the Dark Matter Block to its raw form is to use an Energy Condenser or a Transmutation Tablet. One Dark Matter Block has the same amount of EMC as one Dark Matter because you get 4 Dark Matter blocks from 4 Dark Matter.