Firstly, you need to know this is NOT a Blackboard support forum. There are better places, I think, to search for that. It would probably be best to contact your school's IT department or else a professor with such a question.
I will say, though, I took a few courses last year, and we used Blackboard. I do recall there being a way to save posts rather than making them actually visible. Once it was saved, it was NOT very obvious as to where to get back to the post. So you may have to poke around a lot to find where the saved posts are located, assuming you ended up saving accidentally instead of posting. I forget the details, b/c we didn't use that portion often except for one or two of my classes.
"But thanks be to God, which giveth us the victory through our Lord Jesus Christ." 1 Corinthians 15:57