VoiceThread is an online collaboration and sharing tool that you can use within Blackboard. It allows you to share images, documents and videos to which others can add audio, video or text comments. You can draw on the screen while you are talking to highlight what you are talking about.
You could use VoiceThread in a number of ways in your teaching, for example to provide audio feedback on student work or hold asynchronous (not real time) audio discussions with your students. The VoiceThread Higher Education page has examples of how it can be used.