The previously noted solutions of the dark page is great, but they lack the ability to have the address bar auto-focused, which to me is a non-starter. I'm used to being able to immediately start typing a URL or a search item right off the bat when I open a new page / tab. This is such a good feature that I went ahead and submitted a request for Apple to add this feature, but I think in the meantime there may be a clunky workaround.
While I did not find any blank page html or css floating around the
Safari.app bundle unfortunately, I realized that we could use a macro app like SimpleKeys, Keyboard Maestro, or another such app to create a double whammy where we could set the default blank page as noted previously, ie:
Edit ~/Library/Safari/DarkBlank.html (and set your blank page to this):
And then set a macro to initiate a (command-t,command-l) or (command-n,command-l) in order to create a new tab or window with the address bar in focus. You'd have to get used to a new shortcut to do this, but you'd get the expected behavior.
At any rate, I hope that there is ultimately a better solution put in place by Apple themselves, but if you want address bar focus, this is probably the best we can hope for at the moment unless someone is aware of another way to hack the page (perhaps via NinjaKit or EasySIMBL).