is this a scale???

[maarkr]

OK, in like a 12 bar blues in D, the guitar or bass plays this descending scale d.d.c>b.Bb.a.g.F#.e.

I looked at all kinds of scales in D and could not find one that fit... is this just a classic riff from old blues songs?

[JonR]

The first part of the descent (D-C-B-Bb-A) is a classic blues turnaround.  The chords associated with it could be:

D - D7 - G - Gm - D/A (- A)

The key is still D major, the C and Bb notes are chromatic passing notes.  D7 is what's known as a "secondary dominant" (V of G), and Gm is a "minor IV" chord.  They exist here purely to provide that descending line.

There are many variants, with other chromatic descents, or sometimes with rising lines.

In blues in any case the 7th of the scale is generally flattened.  You'll get a C on the D chord even when it's not leading to B on a G chord.  And the only time you'll hear a C# is on the A chord.  You'll probably also get an F natural, certainly on the G chord and maybe even clashing with the F# on the D.

IOW, don't expect blues to obey classical theory rules - even though the above sequence does.