Stirring the top of the mash will help the lautering by closing any channels in the mash and forcing the sparge to flow through the entire mash evenly. But, dark on top? Maybe they are referring to the tendency of adjunts to stick the mash due to poor starch converstion. If you put them on top and stirred occasionally you'd avoid this part of the problem, but would the starch convert on the way through the pale?
That said, every brewery I've seen mixes the grains as completely as possible and stirs or rakes the top during lautering.
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