Black cleaver states that when you kill a unit, you gain 60 ms. This is this case with normal boots, however, if you have swifties, the number next to the movement speed stat only increased by ~50. I tried this is Jax and Darius so far.
This is because movement speed has 2 soft caps, one a 415 and another at 490, and once you reach one of these soft caps, every extra point of MS you get has reduced effectiveness.
To be exact, once you reach 415 MS, every additional point you get is reduced to 80% effectiveness, and once you reach 490 MS, every additional point is reduced to 50% effectiveness.
