What will different ports do?
To make it simple, ports are pretty much slots for incoming connections. Blockland used to use 28000-28030, because that's 31 slots - designed for 32 players with one being the host (at least that's what I've been informed of). I'm not sure if it's because of new router technology or Blockland does it for you, but you only need to forward 28000 now. So, by setting it to, say, 29000, Blockland can also accept connections in those slots.
If you tried running two servers on the same range of ports, it won't work because those slots are already being used.