you get a blank screen because there's no connection (your client is not connected to a server) and you have inside your code some line that is preventing level load until client is connected.

I had same problem and the following worked for me:

#for server mine cmd line looked like this: >> -sv -cl -port 2300 << and i forwarded that port. if you don't know what forwarding ports is then be free to ask, i'll explain it to you.
#for client mine cmd line looked like this: >> -cl -ip localhost << for testing on one pc and >> -cl -ip somenumber << for online testing


hope this has been hellpfull to you.