Colors of wire differ between what electricians use and what electronics types like me use.. And the difference can be as great as black and white
To me (electroinics) and the person who designed your converter BLACK is either negative or chassis ground (Not the same as earth ground)
WHITE, something else
To an electrician WHITE is "Neutral" and bonded to earth ground at some point.. Black is ... Something else (Hot)
So for a white wire to be hooked to the BLACK terminal on the converter... normal.
Now for the blue wire.....
Parallex bought out Magnatek, I know the older 6300 better than I do the 7300 but this I do know
The 6300 had two sets of POSITIVE wires, Red and blue
one went to the battery and to sub circuits like the radio which work best on clean power.. The other just went to sub circuits like lights that do not mind very nasty power (Rectified AC with no filtering) I forget which was which.
But again, if the wiring panel was originally designed for the 6300 or something like it, then both red and blue connect together.