I am no colour expert but I can't think of any reason why you'd need to process Portra differently for dark skin. It has to be designed to capture dark skin well under normal processing, surely?
What did you take your exposure from?
If you are not comfortable with 1/30 or even 1/60 then I think that Portra 400 will still produce good prints rated at 800.
You may be worrying unnecessarily.