Ha!
That was a pre-coffee answer...

Let's try that again, post coffee...

At one point, there was a turn-in. NPCs in black hooded robes were lurking around Haven, trading blackrock for all manner of goodies (some good goodies, at that).
Then, they vanished, and Haven was destroyed.

Now, the only use for blackrock is the Armageddon spell, which is not that useful.

It was mentioned in a recent House of Commons, though, that there would be a use for Blackrock.

Sprite - *Lariat* Can you tell us anything about the upcoming content involving Blackrock? Will it be related to the crafting plans, by chance?
Draconi - Hi! Have you been hoarding blackrock? Do you hear voices? Talked to any wisps lately? Well, now that you've helped rip a few holes in the very fabric of reality, be sure not to miss the Mad Mage event, where we'll be trying to find even more!
RicTheHippy - [insert mad cackling and sounds of lightning here]

and...

Huh... Can't seem to find it...
I'm sure there was a mention somewhere that there would be a use for the horded blackrock. But, it carried with it an ominous "not what you'd expect"