Blackboard always has a confirmation screen you should screenshot and it sends you a confirmation email. If you did turn it in and have the email to prove it, go to the professor and the dean of that college. If not, then there’s not much you can do. I’ve had issues like this with Blackboard too, but I realized mid-semester luckily, not at finals.

sue blackboard for your entire semester tuition cost, they are incompetent trash and have been for over a decade, back in 2004 i got dinged on a lab homework where blackboard had said there were no assignments and there actually were

Don’t feel bad. Blackboard is designed to make everyone feel like an incompetent idiot, even faculty. “They” have to keep us humble.

Just like TurnItIn. Lost track of how many times I submitted assignments, double checked they had been submitted only to check a couple of hours later or even the next day and find that it is still marked as 'draft' and have to go through the submission process again.