Is it self-plagiarism if I upload a paper in error, delete it, and then upload a modified version again somewhere else?
It's not self-plagiarism. Self-plagiarism involves trying to get double credit for the same work -- mostly in academic publishing, where the same author publishes old ideas as new ideas (without citing). In your case, you accidentally uploaded the paper somewhere and then deleted it -- this does not even come close to meeting the bar for "claiming a publication." 

There is a possible concern that you might be wrongly accused of cheating -- either because your professor saw you post the work to blackboard and assumed you were trying to share it with someone else, or because blackboard automatically indexed the paper when you uploaded it, and will now flag it as a duplicate when you submit it. You didn't do anything wrong, so you should be fine - but if you are concerned, you could send a brief e-mail to the professor explaining what happened.