It seems that the Holy Grail for most modern guitar players is complexity, with a mantra of 'more complicated is better, faster is good!'. It's easy to play this kind of game, particularly when we're younger. Our ego takes over and pushes us to want to be a better player than the next guy, constantly making us show off our chops and super fast licks. I too, was guilty of this, and it was my habit to play exciting complicated ragtime blues pieces, often while I talked at the same time! To my way of thinking, I considered a song with just three chords, like most blues tunes have, was just to easy for me and didn't bother with them.
It wasn't until I listened to Eric Clapton's Unplugged album that it suddenly dawned on me. I had been sadly mistaken in thinking that the old blues music was too simplistic. Although I'm not a big Clapton fan, the album demonstrated various styles of playing that I just hadn't come across before. It set me off on a Youtube search for the old roots blues music, with it's associated African influences. I discovered artists like Big Bill Broonzy, Robert Johnson and Son House. Of course, I also found other people that played the ragtime blues variety, which still satisfied my need for complexity should I ever need it!
One particular time, I came across a video featuring Lightnin' Hopkins, the famous Texas guitarist. It was called 'The Acoustic Blues According To Lightnin' Hopkins' and about half way through he was sat on a sofa in his house gently playing a song in E. Although he only played a few notes, the sound sent a tingle up my spine and I was determined to learn these powerful finger picking techniques. Of course, the only way to do this nowadays is by listening to the old so-called 'race records' again and again, studying the notes and guessing which string was being plucked.
It was obvious that the thumb was massively important to the fingerstyle techniques of these guys. Normally, it keeps the pace and is often heavily damped with the picking hand palm, so that it sounds more like a 'thud' than a musical note. It can also alternate and make the music more like ragtime, called the Piedmont style. Other times, the thumb moves over to the treble strings to help produce single string musical runs and increase syncopation. This latter technique is completely opposite to the classical way of teaching guitar, where the thumb sticks to the basses and one, two or more fingers pick the treble strings.