You probably printed on a media type that requires matte black, such as enhanced matte.
The driver automatically switches ink types based on your choice of media type.
So if you want to print on Premium Luster paper, choose Premium Luster for the media type, and hit print. The printer will automatically switch back to Photo Black, since that's what's needed for the Premium Luster media type.
Or you can invoke black switching manually from the printer panel itself (i.e., the physical printer unit, not the software driver).