It's been a while since I read the book, but I don't think that's quite right. A market can be a "blue ocean" but be too small to support a billion dollar business. And you can certainly build a billion dollar business in a red ocean, if you just execute better than everybody else.
Now if one was to say "assuming sufficient size, it's easier to make a billion dollar company in a blue ocean market" then I think that would absolutely be true.
Blue ocean does not equal unicorn. That's a mistake the author makes. Blue ocean, by definition, is just an open market. That market can be big or small.