The Ghastly Blood Cross wasn't like the Eight Level Devil Scripture. Once he used this offensive attack, the fact that he had dark Force would be exposed.
The Eight Level Devil Scripture was a physical scripture and was used to heighten his strength and physique, so most people wouldn't detect the problem if he didn't release its true power and reveal the devil pattern special trait of this scripture. Also, Wang Teng had the system covering up for him. Even the dark apparitions might not notice what was amiss.
At this moment, Wang Teng remembered the activation theory of the Ghastly Blood Cross. If he removed the dark Force and only used the evil aura with one of the five element Forces, would he be able to execute this earth-rank technique?
He could give it a try in the future.