Jimmy Page was well versed in a variety of guitar styles. Beyond blues and rock, Page was fascinated with folk styles, and one of his biggest influences was the British folk guitarist Bert Jansch. Page loved to combine Celtic and Indian influences, so he took the main theme of Bert Jansch's "Blackwaterside", performed as an instrumental adding a tabla and retitling it "Black Mountain Side". Where Jansch's recording of "Blackwaterside" is credited as "Traditional, arranged Jansch", Jimmy Page gave songwriting credits for "Black Mountain Side" to himself. In 1977 interview in Guitar Player, Page admitted, "I wasn't totally original on that. It had been done in the folk clubs a lot; Annie Briggs was the first one that I heard do that riff. I was playing it as well, and then there was Bert Jansch's version. He's the one who crystallized all the acoustic playing, as far as I'm concerned."
[1] Bert Jansch is aware of the influence he exerted over Jimmy Page. In a 2007 interview in Classic Rock, Jansch observes, "the thing I've noticed about Jimmy [Page] whenever we meet is that he can't look me in the eye." When asked to explain, Jansch continues, "Well, he ripped me off , didn't he? Or let's just say he learned from me. I wouldn't want to sound impolite."
[2] The article goes on to point out many other successful musicians who owe a debt to Jansch, and gives examples of others who have closely followed a song originally by Bert Jansch. The most notable of these is "Ambulance Blues" by Neil Young, which draws from Jansch's "Needle of Death."
[3] But where "Ambulance Blues" includes original work of Young's alongside the Jansch influence, "Black Mountain Side" is directly taken from "Blackwaterside."
Recommended Listening
 
Bert Jansch - Dazzling Stranger
 
Anne Briggs - A Collection
Compilations of Original Versions
"Blackwaterside" by Bert Jansch is included on Led Astray and The Roots of Led Zeppelin.