Watch the Black Dog: Ex. 4a online guitar lesson by Andy Aledort from Kings of Rock & Roll Vol. 7: Jimmy Page
This excerpt is basically a one-chord "stomp"-type blues in A, with brief use of G-to-A at the end of the pattern. Page was a master at creating thematic riffs in his solos, and, in this example, I begin with a repeated phrase based on A major pentatonic (A B C# E F#), followed by a phase which alludes to both A Dorian (A B C D E F# G) and A minor pentatonic (A C D E G).
These phrases are followed by "clipped" staccato riffs based on A minor pentatonic, which are indicative of how Page was influenced by the blues masters while developing a sound of his own.