Interesting article. Blackcoin however is not a hybrid PoW/PoS — it was for the first 10,000 blocks and then became the first pure PoS coin and has remained so for over 2 years, producing a block around once a minute without fail: https://bitinfocharts.com/comparison/confirmationtime-blk.html
Development to the protocol over time eliminated the coin-age attack vector you describe and the reward became static, incentivising everyone to secure the network. See the white paper for PoS v2 http://blackcoin.co/blackcoin-pos-protocol-v2-whitepaper.pdf.
Loads more has gone on since then, I’d ask on the Reddit if you had any technical questions and encourage you to try PoS in Blackcoin, it’s almost free at current prices, or hit me up (patcrypt on reddit/twitter) and I’ll send you a few coins.