
Check out the big thread about it
http://forums.overclockers.co.uk/showthread.php?t=18275663
Though:
Mining blocks in corruption - use dynamite or purification powder. Eventually you can get a nightmare pickaxe or better to mine it.
Shadow orbs - in the corruption, at the bottom of the shafts.
Check out the TerrariaServer.exe inside your Terraria folder (in steamapps)
How do I get the Nightmare pick?