Hi,
In the WMP 12, it is by design set to blue color, however you can follow the below steps to check if it helps.
Before editing the registry, please take the backup of registry incase if the steps goes wrong, you can restore it to the previous settings.
To back up the registry, follow the below link.
http://windows.microsoft.com/en-US/windows7/Back-up-the-registry
a) Go to start search and type “regedit” without quotes.
b) Navigate to the following registry key:
HKEY_CURRENT_USER\Software\Microsoft\MediaPlayer\Preferences
c) In the right registry value pane, locate the REG_DWORD (DWORD 32-bit) registry entry named LibraryBackgroundImage.
d) The default value data for LibraryBackgroundImage is 6. Modify the value data to other number to change the WMP12 Library background image.
e) Exit from Registry Editor and restart Windows Media Player for new background image to take effect.