Yep, they 'blowed' it up real good.

It wasn't an asteroid, it was a comet, and they didn't 'blow' it up, they sent an 'impactor' into the comet at 23,000mph to study the material released from the collision. The probe was the size of a beer barrel, the comet is the size of Manhattan Island. Not much chance of the collision completely destroying the comet, wouldn't you agree? Here's a
LINK to the news story.