Have same problem and you will waste Your time with cleaners..... I've tried with better than you can buy. You can get some off with detergent, but the black stains remain. Evidence for me ( 27 yrs as a chemist for a large washing powder company) it's not a cleaning 'detergent' problem. Its a dye transfer problem - from the dyes/pigments in my leathers into the surface of the seat.
So so either 'solvate' if off' ethanol/IPA and heavier hydrocarbon solvents - (and in doing so risk taking yellow with it) or bleaching it off with whatever. (ditto issues mentioned ).
Or buy another (lower) seat (as I did) live with the occasional scrub to make her presentable and ping the OEM one back on when you sell it.
Oh, maybe try steam if the black dyes are more soluble at High T than the yellow.
(Caveat above obv and I accept no liability if you get Your misses X5 steam cleaner out and it all goes pete tong).