First of all, all kinds of people gathered in the Black Market, which could bring useful information to Zhang Ruochen.
The fight between the Ancient Race of Prison Guardians and the Immortal Vampires would surely astonish the whole Yuan Mansion, and even the State of Tiantai. So messages would be exchanged frequently in the Black Market.
Therefore, even if Zhang Ruochen didn't go to the Pluto Sword Tomb, he could still get the information he needed.
Second, Zhang Ruochen went to the Black Market to buy the Divine Origin Pill of higher levels, preparing himself to break through the Third Level of Half-Saint.
Only with higher cultivation could he win more initiative. Otherwise, he would always be on the defensive of the Imperial Government and the Immortal Vampires.
Once he reached the Third Level of Half-Saint, he would deal with Half-Saints at higher levels with more ease, instead of being kept on the run.