I have noticed that there is a sell wall slowly pushing price of Blackcoin down on Mintpal. It is worth of 20 BTC and it is gradually going down.

Maybe it is related to argument between cryptorush exchange owner and BC developers. Somebody who seemed to be from this exchange promised revenge and destruction of BC.  If it is true, it would be ugly, even criminal acting. BC trading is closed on cryptorush, he could "borrow" coins from there to manipulate price.

Other explanation is that it is just a whale trying to buy more BC by pushing the price down.