This to me was always obvious. They wanted a way to define special targets that had specific behaviour but didn't want people to get confused when they tried to call something "blank" and then it created a new window. Having a special character at the start of your name by default is extremely uncommon so you're very safe if you add a special character at the start of the name for special purposes it won't conflict with someone's actual desires. "_blank" isn't the only one. "_parent", "_top", etc also exist.