Depends upon what was used to apply the black matte finish and white lettering.
If it's decals on top of the black handlebar & stem, heating them up with a hairdryer will let you peel them off.
If it's black-anodizing on the aluminium and white paint, then paint-stripper will remove the white paint and leave the black alone. However if both black & white are paint, no easy way to remove the white without damaging the black. Personally, I would just sand both stem & bars and spray black over both.
Quick and simpler way would be to overwrite the white with a Sharpie marker. May not be a clean as re-painting, but certainly easier and quicker.