In a story titled "Prelude" written by "Katherine Mansfield", I came across the following paragraph:
After tea Kezia wandered back to their own house. Slowly she walked up the back steps, and through the scullery into the kitchen. Nothing was left in it but a lump of gritty yellow soap in one corner of the kitchen window sill and a piece of flannel stained with a blue bag in another.
Please somebody shed some light on the meaning of "a piece of flannel stained with a blue bag".