I was talking to Aaron at River Run Anglers the other day, and he said that he used to use Firewire for his Articulated patterns until a bass fisherman came into the shop one day and showed him that if you keep twisting the stuff, it comes unravelled.
Personally, I use the same material I use for my Musky leaders, Spiderwire Saltwater Fusion in 30 pound. If you don't believe this stuff is super abrasion resistant and super tough, try cutting it.
It comes on spools and is used for bait casting rigs.Or you can use black backing. I don't like the fusion or spiderwire etc since it frays and breaks in this application. I have used other colors of backing and then touched them up with a black marker.
Go to almost any grocery store and they have fabric dye, I think it is called Ritz or something like that. Just follow the instuctions. I still have an aluminum pot my daughter used to dye a sweater and it is still black and that has been 10 years ago. No I don't use it to boil corn on the cob. More like corn beef and cabbage. The Irish don't know the difference after a few dozen rounds. My kind of people.
The Spiderwire Saltwater Fusion is coated with a protective coating on it and doesn't fray, it's designed to take a beating from rocks and other saltwater applications. It is also a dark green color, but I don't think it matters about the color for an articulated leech, you can't see the string anyway. I've also used dacron coated black with a permanent marker, but I like the Saltwater Fusion for it's durability.