It is better to set the colours by using the system preferences
application. The key you are looking for is
selectedTextBackgroundColor.
But none of the default colours of GNUstep is blue, actually all of
them
are some sort of gray. Most likely you are already using a GNUstep
theme
or have your colours changed in some other way.
If you insist on the defaults method of setting a colour here is the
command for you:
defaults write NSGlobalDomain selectedTextBackgroundColor "0.0 0.3 0.8"
or
defaults write NSGlobalDomain selectedTextBackgroundColor
"{ColorSpace=NSCalibratedRGBColorSpace, R=0.0, G=0.3 B=0.8}"
But you really should use system preferences instead.