Toning using colour couplers requires a normal rehalogenating bleach - ferricyanide and bromide, then after washing redevelopment in a colour developer with the relevant coupler added, for blue it's αlpha-napthol.

You can either leave the coupler dye plus the silver image or bleach the silver. Bob Carlos Clarke used Tetenal's dye coupler kits but they are easy to make up from scratch.

Ian