Port Forwarding just opens connections so people can talk to your computer and recieve information. It's a bit of both the ports you forward and the program. Ex, you forward the port 28000 all that does is allow Blockland to send/talk information to other computers, nobody is uploading to you, just downloading information from you. Blockland also handles everything Server Sided (like almost every game) so meaning people can't attempt to put stuff on the server or your computer because it just won't work.
The only way port forwarding could allow you to get hacked is if you have an FTP. FTP is a file transferring protocol where you can send and download files from a computer anywhere (you set up which files and the computer etc) FTP sends files over in the clear without any encryption (encryption can be added but by default it's not). The default port for FTP is 21 but you can change that, if someone with a lot hacking knowledge knows the port and how to hack into it they can potentionally get that information your sending over, that's basically the only thing they can "hack" at least at my understanding
You won't be using FTP for BL obviously, that was just an off topic example of how someone could use ports to "hack" files, not your computer but just files.
