Well it has been many years since I dyed traps, but when I did, I boiled them to get the oil off. hung them outside till they rusted, and then soaked them in a "willow" tea.
I got some willow branches cut them up with secutars [hand pruners] adn dumped them into water. bring to a boil and drop the traps in. I left them for a few days and when they came out they were black from the tannin in the willow.
This is an excellent time to gather some fine branches as thye are just leafing out here. T he tannin is in the bark, so all you want are the very small diameter twigs to about 3/8 inch.

I'm sure Halford would have some commercial product, but this worked fime for me.