Thank you for your explanations. But I thought there was some very old, very traditional reason.

I first know a folk song 'Blue' (you good dog, you...). And then I once (long ago) read a story of a french king (don't ask me his name) who used to have a cross burned into the tongue of everybody he heard using the Lord's name to swear. So the people who considered that procedure a little painful began to replace 'Dieu' with 'Bleu' - the latter being the name of the king's dog, and it rhymes. Therefore 'Parbleu' and 'Sacre Bleu'. Also some kind of language larf, although not in english.

Did french kings still burn crosses into tongues when australians began to call red-haired people 'Blue' (I thought this was a general australian expression for everybody) so maybe a red-haired dog was named so? Or were there more real blue dogs in former times? (I even don't know one of them, also I once had a brown dog - never thought of naming him 'Brown'.)

Still ?????, sorry.

Joerg