I don't see anything wrong with that and it will be still be black.
First some facts:
1-Though they are all three listed, you don't know the amounts. So the PBr7 could be a small amount.
2-Mars black dries fast, good for underpainting, is very opague, strong tinter, can vary in the shade from bluish to black.
3-Lamp black dries slow, not good for underpainting, opague, strong, can vary in shades, usually warm brown shade.
4-Artists who use black to make grays often use a PBr7 pigment to neutralize a biased black
5- Names are fickle monikers, the pigment numbers are more descriptive and that PBr7 will range in shade.
If you mix the three in the right proportion you can get a middle of the road,
drier and neutral color black that can be used for underpaintings, desaturating, whatever, because it is middle of the road.
This is my guess, only Weber knows for sure and you could call a service rep if it matters enough, good luck.