Hmm. There are a few Blodau this/that/tother but I don't see Blodau'r Gorllewin in any books to hand - modern ones, or Nicholas Bennett Old Welsh Airs, or in Cass Meurig's republication of the John Thomas collection. Might be in Llyfr Cerdd Tannau (1913) but it isn't indexed so I'll have to go through it. If you want provenance, I think we'd need to check probably with Robin Huw Bowen or, outside chance, Phyllis Kinney (she's more 'song' related).

I have heard it, I know I have. Can't call it to mind ...

sian