It's supposed to be an adaptation to a cold climate. The closer to the equator a bee is then the lighter it becomes. Dark bees are meant to be better able to absorb the sun's rays and hence can fly in cooler conditions. This means they are more successful than light ones and the advantage that confers means they will survive and breed more. The Aussie bees that Andrew was talking about were in the mountainous West of Tasmania where the climate is better suited to black than yellow. I understand though that the Amm queens in Tasmania are quite light.