Ah, I thought it might have been the Red House session you were talking about.  In fact, that particular version of the tune came from an undated manuscript book where it was titled Johnnies Grey Breeks, and written out in C.  We play it a fourth lower because it turned out that way (and nobody really enjoyed it in C); it's usually played in D elsewhere, though.  I'd go with the consensus here; just play a note that fits and don't worry about that B.  Quite a few of us do that sort of thing (especially me, with tunes I haven't finished learning) and the result, when it works, is a nice textured, "arranged" sound.  In some cases, I've become so attached to my improvised second part that I haven't bothered learning the real tune at all.