I am familiar with basic music theory and know there are 7 notes in the major and minor scales. However, sometimes in Jazz or with the Harmonic minor scale, you might add an extra note here or there. What I'm wondering is if there are any standards when it comes to 8-note scales (8-note major/minor scales specifically), or if not, what you consider to be the best added note to the standard 7-note major/minor pair. By pair I mean that the C major scale is also the D-minor scale, so this 8th note would work well with both cases.
It's going to be a chromatic note, and the one extra that best fits both major and minor scale notes could be ♭5/♯4. It's used a lot in blues and jazz - in blues it's known as ♭5, whereas in jazz it tends to be called ♯4. Love to know why, as it's the same note pitch. Possibly because in blues, the 3, 5 and 7 all get flattened from the normal major notes, so sticking with flats.
It's the tritone from the root - in key C, it's F♯/G♭, and lies just between the other two prominent notes in that key - F and G.