Mihawk implied in MF that there was(used to have) a gap between him and Whitebeard
A Whitebeard who in his prime was Roger's equal. So Roger not having a black blade doesn't really help in the argument that having a black blade means you're stronger than other users wielding a blade
Imo the black blade stuff is heavily related to the color of armament(with the highest lvl of control allowing you to turn a blade into a black blade), which coincidently is Zoro's haki specialization
While as a conqueror, the one who conquered grandline, Roger specialized more in color of conqueror, which coincidently is Luffy's specialization
A Whitebeard who in his prime was Roger's equal. So Roger not having a black blade doesn't really help in the argument that having a black blade means you're stronger than other users wielding a blade
Imo the black blade stuff is heavily related to the color of armament(with the highest lvl of control allowing you to turn a blade into a black blade), which coincidently is Zoro's haki specialization
While as a conqueror, the one who conquered grandline, Roger specialized more in color of conqueror, which coincidently is Luffy's specialization