If anyone has any solutions (other than portforwarding), please comment.
Don't think there is a solution without port-forwarding.
Port forwarding a game on your router is telling it to "if it sends a message of this game from the internet, send it to this computer"
If this is not done, a router doesn't know where to send the ping message, so it rejects it.
Blockland is programmed to bombard the router violently with punchpackets so every connected computer get a signal.
This way, people can still join even if it isn't port forwarded.
There is really nothing wrong you can do with port forwarding, it is completely harmless.