I'm surprised your black leather is slippery. Mine actually has more grip than the Caudabe Sheath, so that (and the fact that the buttons are amazing and the bottom of the case feels better on my pinky finger) is the reason it has been my daily driver.
I’ve tried the Sheath, Apple Leather, and Mujjo, and I agree, the Apple Leather is the most comfortable on the pinky. The Sheath is okay on the pinky, and the Mujjo kills my pinky. I wish there were other leather cases that covered the whole bottom and were comfortable on the pinky, because the corners on my Apple Leather do pop off. I even exchanged, and the second one pops off too. So I glued the corners with Elmers stick glue, and a corner still popped eventually. Now I tried putting even more glue and will see how it goes.