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.
1. There is no update from Excoin recently.
2. However, I'd not say BTER had it coming. Withdrawal for many coins are still disabled, and trading is still impossible.
3. Now it make me ponder that maybe adding NBT to Excoin was a bad decision.
A post from NTB forum just revealed that CCEDK was hacked. It seems that all the three exchanges have NBT listed, and NBT is still closed source.