It depends a bit on what kind of hot water boiler you have.
Most hot water boilers are sensitive to cold water "shock" where the entering water is too cold, and the temperature gradient accross the boilers is too great. If this happens, you can have very expensive boiler repairs caused by the temperature gradient and warping/twisting of tubes and other boiler componets.
I am guessing that that part of the circuit is open because there needs to be some recirculation of hot water. Even if hot water recirculation is taken care of locally at each boiler (and not diagrammed), it would seem that the boilers would contribute as much heat as possible, and only excess water demanded by the pumps (that the boilers could not supply) would come through the "black" section of the header. I would guess that the boilers would not like *extra* cold water drawn through them by those transport pumps.
One way to raise the temp in your transport loop, when the boilers cannot keep up, is to use variable speed transport pumps. This can be based on the temp of your transport return, and can save a *lot* of electricity.
(Though not increase maximum capacity, if this is what you are after...)