Watermelons that get up to 50 lbs need time to convert sugars uniformly in the fruit.  Black Diamond has a 90 day maturity, but that is just a rough estimate. If you get your melons in the ground in July then they struggle to ripen in the cooler September and October weather. Muskmelons and cantaloupes don't need as much time. I would suggest transplanting black diamonds 2 - 3 weeks earlier than the melons, and use blackbplastic mulch and or low tunnels to advance your black diamonds with heat. Or choose a watermelon that stays smaller with a shorter maturity time. Aim to get an early maturing cultivar in with your other melons.