I have a macbook late 2008.Yes iv tried safari chrome and firefox still doesn't let me download i still get a black screen. i downloaded it from the apple app store.Which Mac do you have?
Have you installed Mountain Lion or just downloaded it?
Does the issue happen in all browsers? Ie have you tried Safari/Chrome/Firefox
Also are you sure it's not downloading? I have had a lot of download URL's that took me to a blank page but if you look up in the tool bar for say Safari, there is a Down Arrow and that is showing a progress bar.