What is the difference between target="_blank" and target="blank"?

In short, use target="_blank" it always open a new window but use target="blank" it will open a new window at the first time and this window will be reused after the first.

blank targets an existing frame or window called "blank". A new window is created only if "blank" doesn't already exist.

_blank is a reserved name which targets a new, unnamed window.