Beyond the fact that the blood is not the result of drainage, there are several reasons why the blow had to occur while she was alive and not after death:
1.) Early brain swelling was already evident according to the autopsy (description of gyri)
2.) the hemorrhaging itself
3.) contusions on the right cerebral cortex
4.) contusions on the scalp
So, as far as I can tell, it seems pretty obvious she was alive when the head trauma occured. From everything I've seen all the experts who have spoken on it seem to agree that she was alive when the blow occured.
BTW
The head wound information is in contrast to the strangulation information, which does not definitively tell us, IMO, that the strangulation occured while she was alive. The petechial hemorrhaging on her skin can itself be due to head trauma and the 'dead ringer' for strangulation, petechial hemorrhaging of the whites of the eyes, was not present, according to the autopsy. The violence of the strangulation, by itself, does not speak to whether or not she was alive or dead when it occured.