Adobe Acrobat Reader X requires users to accept a license agreement before they can start using the software. If a user doesn't accept the license agreement, then when they try to load a PDF in their web browser, they get a black screen that may or may not contain a blue scroll bar. If this happens, click on the black screen and you should be prompted with the license agreement. Read it over and click Accept to view the PDF.
The other way to get the license agreement to appear to accept it is to start Adobe Acrobat Reader X directly from a shortcut on your desktop or within your Start Menu.