I always have this worry that a employer may have blacklisted me from an interview that did not work out too well. What signs would I notice if I have been blacklisted by a company?
First, it is unlikely that a company of any size would bother to maintain a blacklist (other than people that had been previously employed by the company that didn't work out). An individual manager or group might recall that a particular candidate did poorly in a prior interview and decline to interview that person for a new position but that isn't particularly common given both the turnover in organizations, the number of groups that would have open positions, and the interviewer's tendency to forget all but the most awful interviews.
If you were, by chance, blacklisted, there wouldn't be any signs. You simply wouldn't be called for an interview. But since the vast majority of applicants for any particular position aren't called for an interview, you would never realistically know if you weren't called because of some deficiency in your resume or because of some blacklist.