You can add the following to the Button section in Black.conf
FocusedImage_L = "Black.png" ( 0, 175, 50, 50)
FocusedImage_M = "Black.png" ( 50, 175, 100, 50)
FocusedImage_R = "Black.png" (150, 175, 50, 50)
You can use the tab key in the program to change the focused widget (in case you have more buttons) and you may want to call button->focus() to have it focused when the program starts.
Just know that changing the focus with the tab key doesn't tend to work very well when stuff like child windows or panels are involved.