The BMW medium blue seats were not really working with the look I am aiming for, so I bought a leather re-dying kit on line from thescratchdoctor.
Seats out, leather surfaces cleaned with leather prep and alcohol from the kit. Bit of light sanding. 3 coats of black pigment with a sponge, then 2 of satin leather finish sponged on. Few days drying, then back in car.
200 miles done since.
I'm really pleased....they have come our far better than I had hoped for. Has retained some of the aging / crinkles / wear lines but everything including the stitching is now black.