Both Bter and Excoin exchanges have been hacked recently. That caused a direct downtick but it on the long term it also means a sustained drop in trading volume for BLK.  

Bter had it coming but Excoin is a blow. That one was part of the Black network. Apparently they got a lead on the suspect though so there's a chance of Blackwavelabs being back in business eventually.  

But yeah, there's your explanation for the drop. The price is down to external culprits but still ones that weigh very heavily.