Port forwarding is no more dangerous than loading a website. All internet connections go through a socket which is bound to a port. The difference between connecting to a server and hosting a server is that you have to open up a spot on your computer for them to connect to. When this port has no socket bound to it, no connections will be able to go through. It is impossible to be hacked by forwarding ports for Blockland.
Furthermore, it is not required to host servers. UPnP support was added a while ago. Enable that on your router.