Are we talking tough as in hard to do, or tough as in a durable finish? If you are interested in technique, see my post on "streaky rust blue" in this category. It isn't hard to do, just labor intensive. If you use Laurel Mountain Forge rust blue, use very little pressure when applying the solution & be sparing with the volume so it doesn't accumulate or run. The directions say that, & they really, really, mean it
. I thought I was doing it correctly, but wasn't quite gentle enough in applying the LMF. 99% looked great, but a couple of places got streaky. Mark Lee's Express Blue is a hot rust blue like Birchwood Casey Plumb Brown & seems to do as well. I made my trough for apply boiling water out of PVC pipe with end caps, & cut open a slot on the pipe with a band saw. You can then immerse the barrel in water that you boiled on the kitchen stove or a camp stove in the shop. It probably would be better to actively boil the parts for seven or so minutes, but that takes a metal trough that requires more cost and fabrication. I'll be building one soon.
Roger B.