Blackcoin is a virtual currency that works on a peer-to-peer basis. You keep Blackcoin in a wallet, and if you keep the wallet open for people to view, you can earn interest on your holdings. This would be calculated at between 1% and 8% per year.
Blackcoin is always free to use, and is more secure than normal banking because anyone can see the ledger that holds all transactions within it. Additionally, the currency can never be closed because it has no owners. A limited supply also ensures it continues to have value.