Watch the Solo 1-2 online guitar lesson by Matthieu Brandt from Jump Blues
Both these solos are played in the first blues position and feature three different standard riff positions.
Knowing where the tonic of the chord is, will give you the starting point of most riffs. In these examples there is a lot of "playing with the third".
Playing the major third of a chord defines where you are. It is often preceded by it's flat third. This is a great way of letting your audience know where you are.
Note: playing the major third and following it up with the minor third is something you want to avoid. Try it, you'll hear why.