Blue powder has been (perhaps now more correct to say was?) used for at least the last three hundred years to make whites look whiter, but I don't think Clifton could have meant Reckitt's Blue. I believe that trade name came into use long after his death. Sorry - hardly worth commenting on - but what I do think is interesting is the frequency with which Clifton's songs (not always identified as such at the outset) appear in this forum. Lots of feral songs can be traced to a theatre or music hall origin, but some writers and singers seem to achieve a particularly high score in this respect and Clifton must be among the top four or five.