By using a 3rd party app like Candybar 3, its well worth the $29.
search google, then when you want more docks and icons you go here and download. http://macthemes2.net/forum/viewforum.php?id=2
If you want the 2D dock (the same style that is displayed when the dock it positioned on the side of the screen), paste this command into Terminal:
Code:
defaults write com.apple.dock no-glass -boolean YES; killall Dock
To revert back to the 3d dock, paste this command into Terminal:
Code:
defaults write com.apple.dock no-glass -boolean NO; killall Dock
If you mean a custom black dock, like this; you can download the files from somewhere like Deviant Art or Leopard Docks (.com or .net) and then apply them using Candybar (Paid) or Dock Library (Free).