No, you are not mis-understanding, if you want really dark skin to look lighter than it really is then just use the reading obtained from the light reflected from the dark skin, if you want it to look like it should then close down a stop or more.
Originally Posted by hoffy
By doing this, you are not under exposing, you would be 'correctly exposing', if you just use a reflected reading obtained from the dark skin you will over expose as the meter doesn't know what you are metering from and would give you a reading based on 18% grey reflectance, it would in fact assume it was a very dark 18% grey and skew the reading suggested to cope with the lack of reflected light. As you are 'not' metering from an 18% grey but a darker tone you have to make the adjustments to the reading obtained to give you the 'correct' tones in the final image.
If you don't want to take the word of someone who was doing this professionally for 20+ years, you can always go and try it yourself?