I heard that Marc Savoy said that during a seance of Marie Laveau a black accordion player was playing. [...]
Is anybody familiar with that or is that a rumour ?
This is what I found in the chapter about Marie Laveau in Janet Allured/Judith F. Gentry's Louisiana Women:
"Marie Laveau's front room was filled with altars laden with candles, images of the saints, flowers, fruit, and other offerings. Here she presided over weekly Friday night meetings, which were attended by "more white than colored". A core group of her closest followers was always present to assist her. A chorus of young singers, accompanied by an old man who played the accordion, supplied the music..."