I've used Normal Goat Serum and BSA as block for brain IHC with comparable results. While I've used non-fat milk as block in WB, ICC and ELISA assays (instead of NGS/BSA), I imagine it's not the best option for tissue as it is quite a bit "stickier" and may be harder to get rid of from your sections. Other than that, milk does contain BSA and should give results comparable to pure BSA if used at similar concentrations. Another question is how variable is BSA concentration in different lots of dry milk, and how will this affect your repeatability.
Looking forward to hearing from a more experienced experimenter than me, or someone who's actually tried blocking tissue with dry milk.