You can easily make an entire quiz or a single question extra credit.
To allow Extra Credit to be an option in grading your test, go to the Question Settings button and make sure the box for Provide option to assign questions as extra credit under Scoring is checked.
Go to the Test Canvas and click on the points box.
Edit the number of extra credit points you want to be given for this question, click the Extra Credit checkmark, and then click Submit or Submit and Regrade depending on if the quiz has been taken or not.
"(Extra credit)"will now appear next to the points box. If that's not there, the question will be graded as a normal question. If you wish the entire quiz to be extra credit – one that you are creating, one that has already been created, or even one that has already been taken – simply repeat this step for each of the questions.
Question sets and random blocks
The above instructions do not work on Question Sets or Random Blocks. For extra credit tests containing these types of questions, you need to exclude the test column from Grade Center calculations and select don’t show to students. Then, create an Extra Credit column with 0 points possible and copy test scores over into this column.
Making an extra credit assignment
Create an assignment in Blackboard (which will automatically create a column) and then give that assignment a point value of zero. When the student completes the assignment, the points will not affect the other columns but can be figured into the student's final grade.