A few points on improvisation, and on blues and jazz.
Improv. It's been said by witnesses that Beethoven could improvise for hours on end. We can only guess at how many other great western composers had similar abilities. Also, let's not forget that Indian classical music performance combines highly complex, strict structural and syntactical rules with artistic extemporisation. And today, French church organists are expected to be able to extemporise during divine services as well as in secular concerts. A couple of examples of the latter:-
Olivier Messiaen - Improv in church organ concert
Olivier Latry - Organ improv on a submitted theme, during a seminar at Notre Dame University, USA
Is the ability and talent to extemporise - at least on piano - strictly necessary for western orchestral composers? I'd say not. But surely it is of great benefit here and there, now and then, when composing. At the very least, who will deny that switching one's head into the extempore mentality is a very effective antidote to the dreaded "paralysis-by-analysis" syndrome? And of course, the more familiar that one is with theory as well as craft, the more extensive, elaborate and complex one's improvisations are - potentially - able to be.
That said, I seriously doubt if anyone can rightly refute that deliberated, analytically-founded composition is able, more often than not, to exceed in terms of richness and depth of complexity, what extemporisation on its own can produce. I suspect the ideal is a profound marriage of both abilities.
Blues. I subscribe to the theory that the genre of Blues was a originally a discovery made by a few black slaves in the Americas when first encountering and using the European guitar tuned to Equal Temperament. My own analysis showed that uniquely in ET there is a very close proximity (approx. 2 cents) to the 17th partial (the angst-riddled and highly discomfiting "blue" note), and also to the 19th partial (an exquisitely beautiful yet somewhat 'veiled' and hence deeply wistful minor 3rd); neither of which are available in any of the historically mainstream intonation schemas. Furthermore, blues musicians finding that the "blues scale" is amenable to modal usage is surely a significant discovery and innovation in the history of music.
However, as proved time and again, especially by the "Boston Pop Orchestra" surge in the '70s, and by the many "pomp rock" attempts to use symphonic orchestras or ensembles to accompany rock bands, orchestras typically don't intone the two crucial ET blues notes correctly in ET. Instead, it seems most orchestral musicians tend to play in their usual and "more proper" orchestral intonation. Hence we have yet to see a great marriage of orchestra and rock band. Not having done any measurements on this (I just can't find any motivation to do so) I can only surmise that talented blues musicians who use orchestral instruments have learned to intone the two crucial notes of the blues scale according to ET.
Jazz. Never been my cup of tea, so I can't speak much on this. However, it is apparent that as a popular music genre, a good deal of perfidious repurposing of modern music theory (and some older theory - e.g. liberalisation of modal usage) has gone into the jazz genre as we know it, and which I applaud.
But alas, it's hard to get away from the view that blues and jazz genres have both done their dash now. I find it sad and often somewhat annoying when attempts are made either to resurrect these genres without any significant innovations, or, worse, to carry on obliviously as if these genres are still current and widely welcomed among general audiences. But in contrast, both John Barry and John Williams have dug into their jazz backgrounds and popped a few jazz constructs here and there into their great and much loved compositions for film, with superb effect. "Gentlemen of taste", as ever, show us the way.