I've found that when playing the last four bars of the 12 bar blues (V IV I I) with triads (in a typical 70s rock fashion) that inserting a major triad on the flatted third sounds really good as a kind of a fill (playing it for one beat, is that the right word for it?).
I wonder why is that? Both flatted third and flatted seventh are blue notes, but my guess is that that's not why it's good, but because it toys with switching to the minor scale. I've found that it sounds a bit more tense if I then use Isus4 as a fill in the last bar, because it puts emphasis on the third degree of the scale in I, which supports my theory.
1. How is this use of the major triad on the flatted third degree called? What is the Roman numeral notation for it?
2. Where do I learn more about this kind of tricks?