Hi kinsuk,
If you want to spell the dominant seventh chords off of I, IV and V in C, they would be:
I7: C-E-G-Bb
IV7: F-A-C-Eb
V7: G-B-D-F
There is no one major scale that would have all these chords diatonically, or arriving solely from the notes of its scale. The blues are tricky this way, because they employ a different melodic scale than is used to supply the scale for the harmonization of chords. The so-called blues scale is a pentatonic scale consisting of I-bIII-IV-V-bVII. In practice, though, a major III note is used to harmonize the I chord, a bIII is used to harmonize the IV chord (for which it is the bVII) and a VI note is added to the IV chord to serve as its major third. In the V7 chord, the bVII note of the pentatonic scale is most often raised to a major VII note. So you can see that the harmonization of the I7, IV7 and V7 chords do not derive strictly from the pentatonic scale which provides the melodic context for the blues.
And in performance, especially as sung, the pentatonic scale is bent and twisted, with the III and VII notes being the ones most messed with, though a lot of blues utilize the bV or #IV note as well. The characteristic blues sound, though, especially in non-Jazz versions of the blues, is that the melodic material from the source pentatonic scale is utilized and works equally well over the I7, IV7 and V7 chords. There are some blues, too, like a number of Henry Thomas's songs, that employ a major pentatonic scale, too: I-II-III-V-VI-I. For a music of such harmonic simplicity (on the face of it), the theoretical underpinnings of blues are really gnarly, especially with regards to vocal pitch.
All best,
Johnm
