It was to avoid the situation of someone naming a frame “blank” without being aware of it having special semantics, or (I’m not completely clear on the history) avoiding breaking compatibility with existing use. That’s much more unlikely with a name like “_blank”. It’s common practice to separate out names with special semantics from general-use names in a syntactic way like that. There’s also “_self”, “_parent”, and “_top”. It’s an extensible scheme where HTML can add further names with special semantics in that reserved name space.

I now agree that the article doesn’t make a particularly good job in explaining all of this.