The primary purpose of ballast is to increase lateral stability. (prevent sideways tipping i.e. roll over). Tractors have a high center of gravity to start with and it’s only worse with a loader. Rear ballast (filled tires and/or three point weights) accomplish this by keeping the rear tires on the ground AND by lowering & centralizing the CG. Keeping the rear tires down is important because most tractors have pivoting front axles that provide no lateral stability. But even with the rear tires firmly planted, a high load on uneven ground or when cornering can easily tip a tractor with insufficient rear ballast. A good rear ballast should lower the CG and maximize lateral stability. In engineering speak this is increasing the lateral moment of inertia. To do this, weight should be as low and as close to the centerline of the tractor as possible. This is why a ballast box is better than a box blade. A box blade distributes the weight over a long distance across the back of the tractor with a significant portion outside the rear tires. Assuming the center of weight is the same distance behind the tractor and at the same height, a tractor with a 300# ballast box is more stable than a tractor with a 300# box blade.
Pound for pound, the ballast box is more stable and safer than a box blade. I’m not saying using a box blade (or any other implement) is unsafe. Just comparing the relative safety of the two.