Jing is a screen-capture application which will allow you to record your screen and voice and give you a link to the recording. You can use this link to give your students feedback on their assignments in Blackboard.
1. Navigate to the student's attempt in the Grade Center of your course.
2. Use Jing to record your commentary.
3. Add the Jing link to the feedback section for the attempt grade.
Here is a video showing the entire process:
(if this video doesn't work, try this alternative link)
The Jing application may be downloaded from www.techsmith.com/download/jing/
Note: if you would like to give your students audio-only feedback, try this the alternative method here.