True. but modern rock harmony seems to be much more comfortable leaving the 3rd off the harmony throughout (and then playing with it in the melody, between the minor and major third and in between) than what I’ve seen in classical music, where it would be considered pretty static/boring to just leave it out completely. For me, the power chords/intervals are kind of a hallmark of modern music harmony and depart from traditional Western music theory a bit.
Oops, sorry for seeming to post-and-run. I haven’t checked the boards much lately.
Tonality is a very complex and robust system whereby every harmony serves a function based on its level of tension (or lack of) within a key, and this function is always pushing forward towards the tonic. In the purest sense, for music to be tonal not only do the harmonies carry these functions, but each note within each harmony has its own sense of gravity and must necessarily resolve in a certain way.
(for example, in a V7 chord the 7th always resolves down a half step. i.e. in the key of C, the 7th of a G7 chord is an F. In the next chord (most likely a C), that F will become an E. - note: don’t think polyphonic instruments like the piano or guitar; think of every note as a voice in a choir and it’s easier to understand this. Each voice has its own melody moving through time, while all the voices together at any one time make the harmony).
Now I won’t say that maintaining perfect voice leading is necessarily what I meant by strictly tonal, but I just wanted to mention it first to help define what tonality is and how it developed. Though we can’t do the strictest function analysis without strict voice leading, we can still do plenty with just harmonies. (note btw that “tonal” is not a binary distinction but can be describe by degree. There is music that is unambiguously tonal (Mozart, Hayden), and music that’s tonal to varying degrees. Atonality is not simply the opposite of tonality, but it is most often a concerted effort to avoid any sense of tonal center (i.e. - tonic; the one note that has the greatest gravity). True atonality, as it turns out, was not so easy to achieve, and this is what inspired 12-tone music and serialism, which was an attempt at creating a sophisticated system to avoiding any sense of hierarchy between pitches).
Blue notes are not antithetical to tonality because they are not chord tones. They are more like chromatic notes (literally “color” notes). They bend and twist the harmony but they still typically resolve back to a triadic harmony. (apologies - as I go I realize I keep opening new cans of worms and I can’t possibly cover every tangent in one post. But I’ll just mention that triadic harmony is a basic building block of tonality).
Power chords, however, come a lot closer to being antithetical to tonality because without the 3rd there is no sense of gravitational pull to a harmony. The 3rd is really where almost all the action is (and the 7th as well). In quantum physics terms, the 1st and 5th of the chord might be like the nucleus of an atom, while the 3rd and 7th are the electrons.
But can we use roman numeral analysis to analyse music based on power chords or otherwise lacking in features of strict tonality? Sure. But to go back to my original point, this system of analysis just becomes more limited because it was designed to describe music that adheres more strictly to the principles of tonality. Beyond a certain point it just no longer becomes helpful or enlightening to try and describe a chord’s function which is what roman numeral analysis is all about.