A blackout is memory loss (short-term anterograde amnesia) induced by consuming a large amount of alcohol or other drug. The type of alcohol does not matter, but rather the potency of the alcohol, amount consumed, and personal characteristics such as gender, weight, and genetics. There are two types of blackouts: "en bloc" wherein all memories from the period are not encoded, and "fragmentary" - also called "browning out" - in which some memories are not encoded.
According to a 2011 article in The Journal of Neuroscience, exposure to large amounts of alcohol does not kill brain cells, as once was thought, but interferes with receptors in the brain which inhibit long-term potentiation (LTP), a process that strengthens the connections between neurons. This inhibition happens primarily in the hippocampus, a major brain area for learning and memory. In essence, memories are not forgotten but simply not communicated or encoded, though enough activity is maintained in the rest of the brain for people to do things such as talk and drive.
This is why, in The Girl on the Train, Dr. Kamal Abdic recommends that Rachel not confuse herself with hypnosis since her memories may not exist in the first place. However, her ability to eventually recall some memories point to her having a "fragmentary" blackout.