51% PoS attack is possible of course.
I'm aware of pre-computation problem. It will be addressed. (https://bitcointalksearch.org/topic/m.6034634)
Also, my opinion about if there is something at stake: https://bitcointalksearch.org/topic/m.5672995
If you disagree with this, sorry, BlackCoin is not secure for you.
Thanks rat4. I agree with your opinion on the "nothing-at-stake" problem, because (from my understanding) as long as the majority of the miners has an honest wallet software that would sync the forks to one single blockchain then the network is secure. While from a game theoretical perspective each one would slightly gain from mining on different forks, I am actually confident that still most miners will act in favor of the stability of their own currency.