is blackcoin's pos requires you to be online at the network for earning interest,minting ?
Yes, you have to have your client open and leave your computer on, of course. All POS coins work this way. They call this minting, forging, harvesting, depending from coin to coin.
Running it on a Raspi, unless you keep your computer on 24/7 is typically a good idea to save on electricity costs. Either that or you could rent a VM on a server and run it there, which is pretty easy to do.
Just remember to keep a backup of the wallet no matter what.
