What happens during a blackout?

It depends: I am assuming you are referring to fainting. The medical term is syncope. There can be many causes of syncope. Some can be serious. You should see you doctor for an evaluation. It could be as simple as being dehydrated to a warning of a stroke to serious disease of the heart.

Blackout: Blackout from toxicity results mainly in amnesia. Blackout from medical conditions such as cardiovascular results in syncope (fainting).