Not every music style comes solely from the usual tree that is blues > rythmn blues > rock and krautrock is one of these, typically european, strange outlets of music that don't automatically link themselves to an american born form of music.
While rock music was a direct child from the blues, in Germany an entire generation of musicians start to twiddle around with a music style linked to experimental music, endless grooves and free form: we're encountering then long pieces of music which are combining improvisations on steady beats and noise/electronics. And somehow it does go back to tribal music, from which originated blues...Upwards from Can and krautrock comes post rock and ambient.
Krautrock has two major acts representing it at the beginning with early Kraftwerk and Tangerine Dream but Can can be seen as a wilder and grittier form: a solid, psychedelic and somehow groovy exponent of a generation of german musicians who wanted to break away from the american mould by injecting more european roots, classical avant-garde and social/political (silent) comments. One can hear influences from Sly And The Family Stones or Zappa, but mixed with rage, anger and...art.
Can can be seen as a european Velvet Underground who hadn't meet rock music and they do share their relative lack of commercial success and influence on bands: just like the Factory's band, Can will impress and create hundreds of bands, the most famous being Joy Division, PIL or Sonic Youth.
Mute Records is releasing Can's catalog these days and especially a box set called The Lost Tapes, curated by Daniel Miller and Irmin Schmidt, which is filled with unreleased tracks dating back from 1968-1977.
More on Can in this eMusic article: http://www.emusic.com/listen/#/music-news/icon/icon-can/