In the edition W.C Handy's book "Blues, An Anthology" edited by Jerry Silverman, the piona parts are given in actual key. The usual pair of clefs are used for this.

Above the piano part is the melody clef with chord symbols on top. On top of the melody line appears first the capo information eg "CAPO III'. Following this over the bars of the melody line are two sets of chord symbols one set in parenthesis. The ones in parenthesis are the actual chords that would be played with the capo in place. The ones without parenthesis are the ones that actually tell you where to put your fingers.

This gives the maximum amount of information. I think the chords in parenthesis are technically redundant because you can deduce them from the piano music, but not everyone reads standard musical notation.

Murray