Wolfie90, yes there is tannin in tea leaves. Unfortunately, even if you brew 5-6 bags of black tea and let it boil down you still won't get enough concentration of tannin to get a real deep black. It will end up more brownish.
Greg, credits for the original idea go to Trevor Talbert and Kurt Huhn - two exceptional pipe makers - who have pioneered the use of tannic acid and iron acetate for contrast staining. And yes, you are right: Leathermakers use the exact same procedure (well - the iron acetate, not the tannic acid because leather already has enough tannins for this to work out of the box). However, they struggle to get the vinegar smell out of the leather for weeks. Briar pretty much repels the smell, we are quite lucky to have it so. Wouldn't touch this technique if my pipe smelled and tasted like vinegar forever, lol!