The Jamaican flag has black in it, (black, white, green and yellow). Most flag colors have represented meanings, so based on the color ideology (white pure, red life, etc.), but some nations apply different specific meanings to the colors used on their flag. The commonality of tri-color flags really begin at the growth of nations - which is a very late concept in the structure of society. Most controlled regions of the world before the rise of nations, were kingdoms and empires, which are generally represented in heraldry. Heraldric devices precede the concept of the flag. IMO, war banners and heraldric devices even printed to cloth is not the same thing as a flag. Flags were created in the Age of Enlightenment, post Renaissance - as a predecessor to the modern world.

If you're like many (most) RPG gamers, the world setting more or less reflects the middle ages and earlier on the timeline - a period before the existence of flags (by my definition above). If you want to show a nation via a symbol on a medieval map, then you will be posting heraldric devices, not flags.

Otherwise compared to modern flags, your design seems quite apropo - nothing wrong with the design at all. It looks like a flag design to me.