There is currently an issue with opening certain documents within Blackboard courses when using Safari on Macs or iOS devices. When attempting to open or download the item, the browser will bring up a different login prompt but will not take the user's credentials.
According to the Blackboard support site, this is due to some security settings within the Safari browser known as “cross-site traffic.” More recent versions of Safari seem to have this turned on by default.
The best workaround for this issue is to use Firefox as your browser as, traditionally, it has the fewest issues when accessing Blackboard.
However, on a Mac, you should be able to go into Safari's preferences, access the section for Privacy, and uncheck Prevent cross-site tracking.
If you are on an iOS device, go to settings, click Safari, choose Privacy & Security, and turn off the toggle for Prevent Cross Site Tracking.
While this has primarily been an issue for Safari users, this can also affect Chrome if it is set to block 3rd party cookies.