When Black Rock is within the range of a Protector it is impossible to destroy, even with the most powerful bombs.
How to Obtain
Blackrock was a Daily Loot reward once. Any players who received this were able to place blackrock. However, once place, blackrock could only be destroyed by bombs, therefore is unobtainable. Developers can mine blackrock, according to @jsonperl
Blackrock was added alongside the Deep Biome in late 2013.