The blue book is a reference, it’s really just like a dump of the man pages. It tells you what each individual function does, but it would be hard to learn using this. It’s also all available in online help or man pages.
The Programmer’s Guide (Red Book) explains how to string various function calls together to draw a particular effect. It explains how to do stuff with OpenGL in very simple terms and will take you through the entire API with examples of code and function calls in context.
If it’s not clear yet, get the Programmer’s Guide also known as the Red Book.