It kind-of depends on how burned "slightly" is. If you are passing the torch briefly over the surface to color it, a light pass with steel wool or fine sandpaper afterwards to remove any loose surface ash and then you should be able to proceed with whatever finish you were planning to use (Tru-oil, BLO, tung oil, epoxy, wax...). Get the handle sanded to the final grit before torching, otherwise too much sanding after will remove the color, you can use bits of metal as masking if you want stripes, etc., on the wood. If you are planning a heavier burn, the burned areas of the wood will become soft. In some cases the early wood may burn quicker than the late wood and the burning may accentuate the grain of the wood, you can use a wire brush to remove the charred material leaving a possibly interesting texture. Finishing may become a bit problematic as it will be almost impossible to remove all the soot without also removing the color and texture, you will need some sort of surface finish to seal in the soot to keep the soot from transferring when you use the knife, but surface finishes don't usually hold up as well on knife handles as penetrating finishes
