So far, i have only really used my box blade as ballast for the loader on my b6100 (vs other forms of ballast). But that is a somewhat unusual situation because the loader only lifts 500lbs and the 60" box blade at 300+ lbs is already considered 'big' for that tractor. I doubt that 'just' a 300-400lb box blade would let a much larger tractor safely max out its loader capacity.
On my B6100, I intend to crank the loader pressure and perhaps almost double the lift capacity (and deal with the fallout.. lol) and one of the things I did in preparation for that was add 113lbs to the rear of my box blade. In my case i bolted on a loading dock bumper (those stacks of used tire pieces bolted together for trucks/trailers to bump into when pulling up to a loading dock) because I figured it would make a good push bumper too and it was cheap on Amazon. Plus, it makes a box blade work better if it's heavier.
So i'm all for adding weight to a box blade you already have. If you don't already have one, buy the heaviest one in the width you're looking at. But it has major maneuverability penalties vs a dedicated ballast weight, and i intend to eventually build one of those too, for a different tractor.