My Palomino mare has a white/yellow tail, which should be totally white but due to the urine stains is more yellow on the bottom...ick...but I have shampooed it with some success (used that blue wonder shampoo whatever it is). Next time I plan on trying one of those shampoos specifically made for removing urine stains on white tails. Not sure what to tell you for a dark tail though, sorry.
As far as brushing out the tail....my mare's tail is pretty thick as well and after shampooing and during some grooming sessions I spray Laser Sheen through it...rub it through with my hands, wait a few minutes, and then brush it through and it REALLY helps lots with the tangles - seems I can brush through her tail in no time using this, plus it leaves it smelling great and very shiny!
Hope that helps...
Robin and Deva