Dear Bailey, as you rightly point out, black horses can be problematic. I feel the direction you're going in - to do it dark - is the correct one as it offers balance.
I've tried this in the past. let me suggest a thin coat of black oils, then when that is almost dry add the highlights. Find a bluey/purpely/light grey-ish mix you're happy with, then apply to the highlight points sparingly. Practice on a spare 54mm horse like an old Airfix or Historex one you might have lying about in a drawer. See what comes out.
So far though, this is a top job.