As a novice getting the hang of playing any instrument, especially a CBG is very confusing. Playing the instrument in a Blues style is even more complicated if you do not understand its development. I have yet to find a good primer on blues. I am not an expert but the following appears to be valid from all I have read:
The major "D" for blues has a "D" for the I chord, and "F" for the iii, a G for the IV, an "A" for the V, and a "C" for the vii. A minor D7 uses the notes of 1 = D, 5 = A, 3 = F, and 7 = C. The Major D7 uses D - A -F# or Gb - C. the big difference is the F and F#. Tuning was developed to facilitate easy playing of these 7th chords and the major and minor chords.
Contrary to popular belief the earliest known blues is from Appalachia. It is mostly settled by "Gaelic" people of Welsh, Irish, and Scottish descent, and blacks. The experts believe the Gaelic people [which included the Druids] migrated in the far ancient past from India, as did the Gypsies. So first look at other scales related to this heritage.
The Hindu scale is 1 • 2 • 3 • 4 • 5 • b6 • b7. The Ethiopian scale is 1 • 2 • b3 • 4 • 5 • b6 • b7. An Oriental scale is 1 • b2 • 3 • 4 • b5 • 6 • b7. The Spanish Gypsy is 1 • b2 • 3 • 4 • 5 • b6 • b7. Adding to this music style mixture were Italian and Portuguese people who used the Maximilian scale. And in this area were also Germanic and Italian neighbors.
As a result of all these cultures they gravitated toward a compromise scale called the Dorian. The Dorian scale is 1 • 2 • b3 • 4 • b5 • 5 • 6 • b7. Combine all these cultural scales and you have a good 7 note blues scale for riffs and arpeggios. 1 • 2 or b2 • 3 or b3 • 4 • 5 or b5 • 6 or b6 • b7. So, to round out the blues scale you can add elements of the other related scales. Over time the The abbreviated Pentatonic Blue scale of 1 • b3 • 4 • b5 • 5 • b7 was often used.
The African slaves brought to America and the Appalachia a unique "instrument" to teach children to play music. It is the forerunner of the Diddley Bow. On the side of the house a one string device was fixed and was played by "fretting" it with a piece of bone, ceramic, or metal. This "slide" fretting carried over into the Blues style. Tuning was often modified to facilitate the slide style.
A style of chording followed on multi-string instruments to facilitate this slide style and blues tunings. However, real blues is never slide alone but requires some finger fretting. The slide set up allowed for the bb often encountered in Blues.
In Blues the root can be played as a major, minor or 7th, This also holds true for the IV and V. The vi is always played as bvi, the vii as bvii, and the iii as biii. The I for "D" can be D, Dm, or D7, the IV G, Gm, or G7, the V as A, Am, or A7, the vi as Bb or Bbmaj7, the vii as C and the iii as F.
Finally, there is a lot of commonality in all the Appalachian music. Blues, blue eyed blues, bluegrass, and Bakersfield country all share a lot of the elements and instruments developed in the Appalachian area to accommodate these styles. To truly understand Blues requires some understanding of its Appalachian cousins and the Appalachian instruments it originated on.