Once upon a time all we had were leather saddles. No-one it their right mind would buy any other colour but black or very dark brown*.
I don’t know what the rich people did then (people who had derailleurs and alloy rims) but we used to rub our saddles with 3-in-1 oil, top and bottom. The saddle didn’t get marked, but our unlined Harris Tweed undies used to get somewhat stained and shiny. That’s probably why we didn’t need those bottom lubricants that seem so popular these days.
As far as being slippery, we used to regard that as a “good thing” – only fat folk actually sat on the saddle, for the rest of us it was a perch with our weight supported by our feet and arms. You may find wool/linen interfaces better with a leather saddle, rather than slippery synthetics.
*so why have I got a honey coloured Ti Swallow, then? beats me 🙂