The difference between the Luminance slider in LR/ACR and the Blacks slider in Selective Color, as far as I can tell, is that the LR/ACR Luminance slider is a relative adjustment and it's relative to brightness values that already exist. The Blacks slider, on the other hand, appears to simply be a global darkening or brightening without regard to what the starting point was for brightness values was in the particular colour being adjusted.
In the images below, you've got the original then the extremes at each end of the Blacks/Luminance sliders. In the side by sides, Selective Color is on the left and LR/ACR is on the right. The Luminance slider in LR/ACR has done a better job of retaining the original brightness differences.