You can hold video-conferencing office hours via Zoom or Blackboard collaborate, or you can hold them via discussion forum. There can be group office hours where students pop in, or office hours by appointment for one-on-one questions.
The Nuts and Bolts of Conducting Online Video Office Hours
Create a poll or survey to find the time when most students can attend.
Create a recurring meeting on Zoom to be used for office hours. Include the hyperlink that Zoom generates for this meeting on your Blackboard Course Site.
Create an Office hours Discussion forum in your Blackboard course where students post questions.
During your Office hours, answer all questions, even if the students who posted them are not present.
Record your (public) Office Hours sessions, and post the link to the Office Hours Discussion Forum so students who could not attend may still benefit from having questions answered.