Anything high in tannins will blacken your traps.
I use the twigs,leaves and catkins from tag alder,and it produces a nice deep blue black finish on the traps.I lay the traps on a bed of the branches in 1\2 a 45 gal drum,then fill with water and cover the traps with more leafy chopped branches,then fill with water.I bring to a good boil,then let the traps set there to cool til next morning when I bring to a boil again and then immediately wax when the traps are hot.Traps like 120's set in trees only need to be done every 3rd or 4th year.
Alder is a building material here for beaver,and not a feed tree,unless starving,so it leaves the beaver traps with a nice natural smell.