I was out at the museum in golden today and I asked the resident historian about the slang term Old Blister.
Apparently it was used to refer to locomotives with ageing, patched boilers when the patchwork began to delaminate and the impurities caused the patches and sheets to look blistered. Apaprently C&S 70 had many such patches on her and given what happened, I would hazzard a guess, one of the blisters popped.