Babe I'm Gonna Leave You (Anne Bredon) was passed from Bredon, a Berkeley folk singer, to Janet Smith, another Berkeley folk singer who in turn passed it to Joan Baez.
Black Mountain Side is a rewrite of Bert Jansch's "Black water side" (trad.)
Dazed and Confused is from The Above Ground Sound of Jake Holmes ['67, Tower Records ST 5079].
How Many More Times is a complex mix, beginning with the Howlin' Wolf source "No Place To Go" (aka "You're Gonna Wreck My Life".) "Beck's Bolero," "Kisses Sweeter Than Wine" and Albert King's "The Hunter" are sources for interludes. The song then goes to two Alexis Korner songs, "O Rosie" and "Steal Away" from On the Move ['68]. Both of these songs originated with Alan Lomax field recordings, the first a prison song, the second a religious song that became a freedom song in the Civil Rights movement.
I Can't Quit You Baby (Willie Dixon) was recorded by Otis Rush in 1958 [Cobra], but his 1966 version on Chicago/The Blues/Today [Vanguard] is a more direct influence.
You Shook Me (Dixon, Lenoir) is a Muddy Waters song.
Communication Breakdown was "inspired" by Eddie Cochran's 1958 song "Nervous Breakdown."
Bring It On Home (Willie Dixon) was originally by Sonny Boy Williamson II ['63].
Thank You has lyrics similar to those of "If Six Was Nine" by Jimi Hendrix.
The Lemon Song is primarily based on Howlin' Wolf's "Killin' Floor" but also takes lines from Robert Johnson's "Traveling Riverside Blues."
Moby Dick morphed from a version of Sleepy John Estes' "The Girl I Love, She Got Long Curly Hair" into an instrumental with a drum solo. The remaining riffs are similar to Bobby Parker's "Watch Your Step" ['61].
Whole Lotta Love (Dixon) is "You Need Love" by Muddy Waters ['62].
Gallows Pole derives from Leadbelly's "Gallis Pole."
Hats Off to (Roy) Harper is a blues amalgam, drawing primarily from Bukka White's "Shake 'Em On Down" and Oscar Woods' "Lone Wolf Blues," but also with bits from Sonny Boy Williamson's "Help Me."
Since I've Been Loving You drew its lyrics from Moby Grape's "Never" ['68, Grape Jam].
Bron-y-aur Stomp borrows its guitar riff from Bert Jansch’s “The Wagoner’s Lad” ['66; see above for more on this folk song].
Tangerine borrows its first verse (and its music) from an unreleased song by the Yardbirds titled "Knowing That I'm Losing You." This may not count, however, since Jimmy Page was in the Yardbirds.
Stairway to Heaven borrows its opening riff from Spirit's "Taurus."
Black Dog throws out blues references over and over, but this is mostly coincidental.

When the Levee Breaks has its origins in Memphis Minnie's 1929 recording, and although it is much more revised, the song actually credits her name.
Rock 'n' Roll opens with a drum solo that sounds like that at the beginning of "Keep a Knockin'" by Little Richard.
Boogie With Stu is a rewrite of Ritchie Valens' "Ooh My Head" and is credited to "Mrs. Valens," Ian Stewart and all four members of Zep.
Custard Pie is an amalgamation of several blues sources. Bukka White's "Shake 'Em On Down" is often cited as the source, but more important are Sleepy John Estes' "Drop Down Mama" and Blind Boy Fuller's "I Want Some of You Pie"; Sonny Boy Williamson's "Help Me" also contributes lines.
In My Time of Dying has its roots in Blind Willie Johnson's "Jesus Make Up My Dying Bed" ['27] and Charley Patton's "Jesus Is Dying-Bed Maker" ['29], but it is more clearly influenced by Josh White's version ['33, recorded as "Well Well Well" in '44 and "In My Time of Dying" in '45]. Bob Dylan also had a variation of this song ['62].
Nobody's Fault But Mine was inspired by Blind Willie Johnson's song of the same name, but is largely original.