First you must be aware, maybe you are, that most terminal emulators come with the ability to let the user define a 16 colors default palette. If 256 colors support is not activated explicitely, most - if not all - terminal emulators will default to those 16 colors.
Because each terminal emulator comes with its own default colors and those can be modified by the user there's no way to tell the value of DarkRed
or even Black
without actually looking at the configuration file or preferences window of your terminal emulator.
Assuming that you are not running in 256 colors mode, you should find where the default/custom colors are defined in your terminal emulator and note their hexadecimal value.
Here is my own colorscheme, in Gnome terminal:

Once you have all the values, you can make a copy of the colorscheme:
$ cp /usr/share/vim/vim7x/colors/darkblue.vim ~/.vim/colors/darkblue.vim
and change all the guibg
and guifg
to the values you noted earlier.
Good luck.