Id try washing the barrels in a very strong solution of washing soda.....dont know if that will work with hot blue but it stops cold rust blue dead....

mix warm water and washing soda in a wall paper trough...make sure all the soda is disolved...submurge barrels and scrub the outsides with a natural stiff bristle hand brush, wash for around 15/20 minutes...let the area under the rib fill and drain numerous times....rinse dry and oil well..

gunut