Not really one for the 'Blues'. I've seen the Moody Blues several times. Do they count? 
www.youtube.com/watch?v=ESER7DFXWiI
Uh, no, they don't. They were never a blues group, even if their original lineup (pre-
Days of Future Passed) leaned on R&B. The original lineup: Denny Laine (lead vocals, guitar, and future member of Paul McCartney & Wings), Ray Thomas (vocals, harmonica), Mike Pinder (keyboards), Clint Warwick (bass), and Graeme Edge (drums). That lineup recorded one album (
The Magnificent Moodies in England,
Go Now in America) and had a smash on both sides of the ocean covering R&B singer Bessie Banks's "Go Now" in early 1965. (In the U.S., the success of "Go Now" earned the original Moody Blues a slot opening for the Beatles on some 1965 tour dates.) When they were unable to follow that hit up with anything else anywhere near as successful, the closest they got being "From the Bottom of My Heart," tension arose in the band and Warwick decided to leave in spring 1966, with Laine following almost at once.
That left Thomas, Pinder, and Edge to think about starting over, beginning with inviting a former bandmate of Thomas's, John Lodge, to join them as their new bassist. Then, near the end of 1966, Justin Hayward (guitars, vocals) joined up. While they concentrated on playing in Europe, where they could earn a good living in live performances, Lodge, Hayward, and Pinder also began writing very different kinds of songs than the original lineup took on. And Thomas mostly put his harmonica aside in favour of the flute, while all five of the new lineup began writing songs as well. (Edge, in the beginning, was more likely to write poems to be read aloud that way over music than to write songs.)
Between that and a quirk of fate involving their British record company, things would change for the Moody Blues rather drastically starting in 1967. British Decca had a new stereo concept they wanted to experiment with, something they called Deramic Sound---and with the Moody Blues owing them a pile from unrecouped advances during their down period, the label all but ordered the band to deliver an album recorded in that concept . . . and, somehow, the band convinced the label to turn away from the intended rock version of Dvorak's
From the New World and let them split the difference, recording a new, original cycle of songs depicting a day in the life, with a full orchestra. The net result was both 1967's
Days of Future Passed and British Decca's Deram subsidiary label, on which would also come such hit groups as Procol Harum (for one album, anyway, before they moved to A&M) and Ten Years After.
Days, of course, became a hit album in England and America; Edge wrote and read the two poems that bookended the album. In England, "Nights in White Satin" became the big hit single, while "Tuesday Afternoon" (edited down from the album's "Forever Afternoon (Tuesday)") became the hit in America that year. (Exactly why "Nights in White Satin" didn't catch in America the first time around is unknown, but when it was reissued in 1972 it became a blockbuster.) And it presented a bigger problem:
Days was successful enough to retire the Moodies' debt to their label, but the label wasn't willing to foot the bill for another round with a full orchestra. Enter the Mellotron, which the Beatles were already using in their music and which Mike Pinder knew more than a few things about---when the Moodies had down periods, Pinder worked in the factory that produced the instruments that could reproduce orchestral sounds through internal tape looping, and he knew enough about them to alter them drastically to fit his and the Moodies' need. (They got nicknamed Pindertrons, in fact.) Beginning with
In Search of the Lost Chord, the Mellotron became a major signature sound for the group---and a temporary headache when they realised a lot of the music they were producing from there wasn't easy to perform in concerts because of the prodigious overdubbing. It seems to have taken them until portions of
On the Threshold of a Dream (Edge once again wrote and recited a poem---essentially, the album's title track---over a sound backdrop) and (especially)
A Question of Balance to write material they
could perform live including Pinder's Mellotron washes.
But you know the rest.
