Watch the Since You've Been Rockin' online guitar lesson by Angus Clark from Hard Rock Survival Guide: Rhythm
Bb? What am I, kidding? Hey, it's a key just like all the others, and it's the relative major of G minor, which is Blackmore's favorite key, so quit crying and man up! (Or whatever the female equivalent of man up is ladies).The chord progressions is I - V(3rd in the bass) - vii - V - IV - I (3rd in the bass) - V. The second section is mostly single note but the implied progression is an Eb chord (the IV chord), to an F# diminished, to G minor, to D7. The F# diminished functions as the vii chord to our new key center which is G minor. So we are tonicizing the G minor chord, which is the relative minor to Bb. This puts us in good shape to play the more riff-based section which is in G minor. Confused? Don't be. Take some time and if you have questions ask your teacher and get it sorted out. This kind of basic theoretical analysis of chord changes is well within your reach and it will help you come up with your own songs, and help you come up with rhythm parts and solos. You can do this.