The Guitartricks course on Rock does indeed mention that it originates from the Blues. Anders actually teaches some blues concepts in the Rock course. The last chapter of Rock guitar level 1 is actually called "Born from the blues."
But therefore it isn't necessary to start with Blues instead of Rock. Anders (the teacher of both courses) does an excellent job to start with either genre when you are finished with Guitar Fundamentals 2. Both build very strong fundamentals and it doesn't matter which one you choose. So instead you should ask yourself: Which genre do I prefer? Based on that you should move on to either Rock or Blues.
But don't get me wrong, BOTH COURSES ARE EXCELLENT! You can't go wrong with either one. Once you are finished with one you can start with the other. In my opinion all the courses (even country) are incredibly valuable.
Have fun!
JJ
