I had attended the drives of companies like harman, aptean in the past and wasn’t selected. Can I apply for those companies in the future through elitmus or am I blocked completely?
If you applied and did not get the interview call, then they will consider you.
If you applied, gave the interview and was not selected, then you cannot apply for 6 months.
What if the same company is hiring for different profile?
If they are conducting a drive for a different profile you can surely apply. (if you are not selected for another profile in the same company.)