I'm pretty sure Planxty did credit Norman Blake on their album. I understand that when the rights to their albums were bought up by Shanachie, Planxty received no further royalties from them. If Norman Blake wasn't paid, that may be the reason rather than any skullduggery on Planxty's part. I'd be surprised if they went out of their way to not pay Norman Blake his royalties but I wouldn't really know.