Has anyone ever dyed their leather trim/piping?

I really do not like the white piping on the jet black leather seats.

 

I haven't dyed piping but have changed leather and vinyl colors before. I expect the piping is vinyl and you could use vinyl prep
and then use a black vinyl dye on piping after taping off the seats. The dye will last forever if it is never touched/abraded (back seats and passenger seat) but would probably wear off in places on drivers seat. But the nice thing about vinyl dye is that it is easy to touch up. Unlike paint, which sits on top of the painted surface, vinyl dye melts into the surface

 

If you are looking at color change for piping, it can be done by a reputable upholsterer.
While you are there you could consider some options. Again, I notice you are in Wisconsin - I took my XJ to Seville Auto Trim (GloTrim) in Ozaukee county. They were finalists in SEMA Launch Pad this year, and ship nationally. Their product is already in some Chevy products.

You can color change and add lighting elements to piping. Reliable product and they have a cool story.