Explain how to construct a blues scale for improvisation on the harmonica, detailing the intervals and appropriate finger placements.

Constructing a blues scale for improvisation on the harmonica involves understanding the specific intervals that define the scale and how those intervals translate to note choices and bends on the instrument. The blues scale, a variation of the minor pentatonic scale, is fundamental to creating the characteristic sounds of blues music and is used heavily in blues improvisation.

The blues scale consists of seven notes. Starting from the root, the intervals are: root, minor third, perfect fourth, flat fifth, perfect fifth, minor seventh, and an octave. So, for example, in the key of G, the G blues scale would be G, Bb, C, C#, D, F, G. The inclusion of the flat fifth (C#) is what distinguishes the blues scale from the minor pentatonic scale, giving it that characteristic bluesy dissonance, and the ability to achieve this note through bending on a harmonica makes it an ideal scale.

To construct a blues scale on the harmonica, you first need to determine the key of the song and then choose the correct harmonica position. For most blues playing, second position ("cross harp") is used. If you are playing a song in the key of G, you would choose a C harmonica, and your G blues scale would be in that second position. Note that a C harmonica in the second position isn't just for playing the G blues scale, but it’s for playing the blues in G, and there are other scales and modes you can play over the key of G.

On a standard 10-hole diatonic harmonica, you can find the root of the scale (G in our example) on the 2 draw note (2D). You can also find the octave on the 6 blow (6B) or 10B. The minor third (Bb) is on the 3 draw (3D) and will need to be bent down to get the note, meaning you have to suck in to get the note, and lower the pitch. The perfect fourth (C) is on the 4 draw (4D). The flat fifth (C#), the most characteristic note of the scale, is achieved by bending the 4 draw note (4D), often by about a half step. This note may require a very specific bend and can take some practice to execute effectively. The perfect fifth (D) is on the 5 draw (5D). The minor seventh (F) is on the 6 draw (6D). These notes, and their bends, will give you the G blues scale on the harmonica and in that second position.

When playing the blues scale on the harmonica, you will primarily be using draw notes. All the notes on the draw side can be used for improvising over the blues scale in the second position, but the bends on 3D and 4D are essential to hit those blues notes. Using a combination of draw notes, and bending those draw notes, will allow you to play the blues scale. A few good examples, when playing in G on a C harmonica are:

-To play the root (G), you will draw on the second hole (2D).
-To play the minor third (Bb), you will draw on the third hole (3D) and then bend it down.
-To play the perfect fourth (C), you will draw on the fourth hole (4D).
-To play the flat fifth (C#), you will draw on the fourth hole (4D) and then bend it by about a half step.
-To play the perfect fifth (D), you will draw on the fifth hole (5D).
-To play the minor seventh (F) you will draw on the sixth hole (6D).
-To play the octave (G) you will blow on the sixth hole (6B)

For finger placement on the harmonica, you will be primarily using the corners of your mouth to get the single notes. You will shape your mouth so that you can target one hole at a time. You will also have to learn to manipulate the position of your tongue to control the bends, and your breath control to make sure the notes are consistent and of a specific pitch.

In summary, the blues scale on the harmonica is not just about knowing which holes to play, but also about knowing how to bend, and using breath control and tongue manipulation. By practicing the bends and familiarizing yourself with the intervals, you can create expressive and powerful solos that make use of the full blues language. The ability to play and improvise with the blues scale on the harmonica is a fundamental skill for any blues musician.