I've been working with leather for a couple of years now, I make all kinds of things. I bought a custom made seat for my bobber and tried to dye it. Here is my experience with it:
Re-dying a seat is very tough. It will never really bind with the leather like the original manufactured process dye will. If you're doing black on black to "refresh" the seat, it will work at first, but keep in mind that rain will just demolish it without resolene, which is messy and tough to get right. The thread will not hold the new dye at all. Your pants will constantly rub off dye during each and every ride so get ready for permanently dyed jean cheeks. Keep in mind that resolene becomes a bit brittle and is meant for leather that doesn't really move. But a seat gets compressed and twisted every time you sit on it, lean, etc.. so resolene will fail here and there.
Anyway, for my seat, I had deep blue and I dyed it black. I covered it properly in resolene. I ended up selling it. Now the guy who bought it is re-selling it, and you can tell that a bunch of my dye has rubbed off, even with resolene, the threads have held none of it, etc.. I think by now the seat is probably where it will stay for a while, but that took a while and the process was not worth it.
This is just my experience. Some people dye their seats and go on with their lives and the experience was a fine one for them. Goodluck. If you have specific questions feel free to PM.