Both BG and David are correct. The Pine Grove Blues are played in dorian mode, or Key of D on an C accordion. It is also similar to the Blues scale. I have played bass with both Chris Miller and Steve Riley playing Tous les temps en temp using this same scale. I have actually developed a couple of tunes using it, but it isn't an easy scale. Technically, you can play it over a major, or minor chord progression, but I don't like it over minor.
Phrygian mode is actually E minor on the C accordion. It is really hard to play any type of melody in this position, but I have heard a couple of Zydeco songs that just have a couple of licks from this position.
I was just working out Boozoo's "keep your dress tail down" and I think it's the same fingering as Pine Grove, more or less.
I had to use Amazing Slow Downer to change the key so I could work it out on my C box(the only one with me for the winter months until I get back home and start gigging again).
I think Boozoo must have used the A key. I just bumped the pitch up.
I do play Pine Grove and it does get easier with time. After a while you'll get to where you can jam/innovate with it.