I use touch-up spray on my horses tail all the time. I brush it really well before I use it (so it doesn't clump) and then spray it in. After I'm done I hold the tail up in my hand for a few minute, maybe swish it around to air dry a little and then it's good to go. I haven't had problems with the tail sticking to anything.
I have also equitone black, but I was disappointed in the results. My horse is brown, but appears black when on BaK, however the shampoo, while it did make him soft and shiny, brought out all of his red highlights.
Quic Black by Exhibitor Labs and Black Horse Henna by Cheval have worked best for me, though I'm not sure they work well on tails. I've never used any shampoo that has significantly changed my horses tail color.