I know that the black chrome, introduced with the Leica M5 and CL, was used in the last M4 prodution (1974-1975), the M4-2, the M4-P, in all versions of the M6 and in the M7 currently available.
The new MP employ the old black paint system, that remember the black paint of the Leica M3, M2 and M4. Is very nice to see, but is also very very prone to scratches. My MP is silver chrome, to avoid this problem.
Ciao.
Vincenzo