Try painting the wash straight into the crevices.
Devlan Mud is very dark brown wash, so once it's in the crevices (opposed to on the surfaces) it'll give a similar look to black just slightly dirtier looking rather than, well, black looking.
You might find black is a bit too dark for shadow grey and Devlan would give a softer look. I use black on my Space Pups, but that's because they're Adeptus Battlegrey which is a bit darker than shadow grey.
But yeah, if you paint directly into the crevices rather than over the whole model (or do the whole model and then go back over with shadow grey, though that will be significantly more time consuming) I think you'll find the differences aren't that huge, Devlan will just be slightly softer and browner giving more of a dirty look while Black won't look as dirty but will give slightly more contrast to the shadow grey. My advice would be to just paint 2 models and observe the difference and see what you prefer.
It also depends how much you paint, as well, you can load up the brush and slop it in, in which case Devlan will look quite dark and black will look very black, or you can paint it a bit softer in which case the black won't look all that dark and the Devlan will be more of a brown tint.
So to answer your question, "it depends"