What can be the home remedy for low blood pressure? should we cardiologist if we face very low blood pressure and it seems everything is blackout?

Depends: Heart failure, adrenal insufficiency, unstable cardiac arrthymias (abnormal rhythms), sepsis with septic shock, orthostatic hypoten, neurogenic shock, cardiogenic shock, and commonly dehydration can cause low blood pressure. It depends how low it is, if low normal and you are not symptomatic you shouldn't get it higher. If you are blacking out you should goto the ed or see a doctor to be evaluated.

Find the cause: If you have low BP with symptoms obviously this should be treated. We need to find out why. Howevere if there is no identified cause then you need to increase your fluid and salt intake. I suggest to see your doctor to better assess that and possibly treat it with medication.