I had the same black streaks on the side, especially where the seat belt is.
I used Lexol leather cleaner, followed the directions, and the black marks came out np
Follow it up with Lexol leather conditioner.
I too have beige leather seats in my 1er
2012 BMW 128i 6MT Deep Sea Blue Metallic
FX-R Bi-Xenon 5000k Bulbs & Bright LED's