This knowledge is convenient for improvising, because it can confirm a modulation to the temporary scale. Moreover the neighbouring white key (for example note f next to f#) is risky because it doesn't belong to the scale.
By paying attention to the relation between the black keys and the scales, we build up knowledge that is useful. But there is more. The scale of Dm can be introduced with the note b-flat, because b-flat belongs to the scale of Dm. It even has a special effect in this case, because it is a note belonging to the subdominant of the scale instead of Dm. The confirmation of this scale is only reached when A7 is played. Note b-flat is a detour: chord Gm precedes A7.
Also scale D can be introduced with note c#, but I assume (and many others with me) alternating between a major and a minor scale more entertaining, although I immediately have to add, that it depends on my mood.
Scale Em can also be introduced with the tone f#, as f# belongs to the scale of Em, but still is less characteristic for it, because f# leads tot scale G in the first place.
In general I see that especially those temporary scales occur a lot that can be introduced with few black keys. If we have another initial scale then C, we don't talk anymore about the black and white keys, but about the notes that belong to the scale or not.
We see these scales occur much in well known songs. We may improvise on the scale of such a song, but we may also decide ourselves when to change the chords and the scales.
That is not as difficult as it seems in the scales mentioned above. During the first lessons of this course we have made melodies on the scale of C, while changing between the chords C, G7 and F on our feeling and without following a prescribed scheme.
The same approach we may use, but then with the extension to the
scales F, C, G, Dm, Am and Em. If we note the scales like this we see
Bb F C7 ..............(from scale C leaded via note b-flat) F C G7 C G D7 .........(from scale C leaded via note f#) Gm Dm A7 ......(from scale C leaded via note b-flat of c#) Dm Am E7 ...(from scale C leaded via note g#) Am Em B7 (from scale C leaded via note d# of f#)Click to hear what I mean (I have no scheme in front of me).