You certainly would not use black buckram if fusing pastel colours each side but in reverse you would use it if fusing dark coloured fabric and you did not want a hint of white to be seen or to lighten the dark fabric, depending on the quality of the fabric. I very rarely use black buckram but recently it was the solution for a trim with red and black fused each side. The edge was finished with a fine braid and I did not want white popping through on edge.