portforwarding is safe as long as you're running safe programs over the port. hosting for blockland/any game is considered safe since the traffic going in will only be for the game; any other connections wouldnt be accepted/data will be ignored afaik
its only unsafe if your computer is infected (the virus may attempt to use the port to communicate and accept commands from outside) or if you let programs with questionable safety communicate over the port.
then again i don't know too much about this, but that's the gist as far as i know. its kinda like opening up a border crossing checkpoint in your router to let internet traffic in; your computer's safety is only as good as your management/control of the checkpoint