Not sure how much this will help - I sometimes use MDF as a ground for paintings. Whether I'm painting in oils or acrylics, I always use some sort of primer - usually an acrylic primer.
I'm not sure Japan black is what you want - that's what was on model T fords etc, before nitrocellulose was developed. If you wiki it, you'll find it sounds a bit complicated for what you want!
I'd suggest a primer of some sort (to seal the very water absorbent MDF) and then some hard wearing paint of your choice...
Thinking about it though... do you want black? Look for "blackboard paint" - several layers of that would probably do you (I wouldn't bother with primer) it's tough, waterproof, and, er, black... :D