Watch the Blues Study in D 5 online guitar lesson by Tim Sparks from Post-Modern Fingerstyle Blues
Use a "slow-hand" fretted thumb to combine the bass line and melody. Getting the thumb into the fretting action turns out to be a big benefit because it allows us to sustain and move the melody around without cutting it short to catch bass notes. That's a big part of making this sound piano-like, because piano players have the luxury of sustaining more notes at one time than we typically can on guitar. Also notice how just a little piece of the bass part is sometimes enough to imply the entire line, which further frees up fingers.