I encounter this often when my connection isn't very stable. I unfortunately use satellite internet (no alternative). What you can try is what Tobei said and also clearing your cache at least once.. what I personally do is just relaunch the game a few times and it goes through. (I use the client mainly) it seems to cache some of the game in memory and the black screen is sometimes a failure to load something. It should work for you hopefully! good luck ;)