Here is another way to look at it:
Port forwarding allows only 1 port# though your router
That port goes only to 1 PC
if the PC is turned off or asleep, its the same as no port forwarding (nothing there to receive the data)
if the PC is on, and blockland is not running, its also the same (blockland is required receive the data)
if the PC is on, and blockland IS running, then its the only thing to receive the data. then look at what everyone else said. blockland is hard to hack.