The Void Dragon King didn't blame Leo for not making a Blood Oath with the remnant spirit of the Dragon God because he knew Leo had no reason to do that.
What's more, he was already grateful that Leo tried to reunite him with his family despite the fact that he brought Leo to the grade 1 Dragon World to kill him.
Moreover, in his mind, Leo was powerful and a being worth following after seeing his actions for the last two days.
Keeping these things aside, the Void Dragon King hated the Dragon God more because he kept going back on his promises and if not for his family, he would have already swore his loyalty to Leo.
Other than reuniting with his family, the Void Dragon God has another goal in his life and it was to take revenge against the Dragon God.
'I hope nothing happens to my family until I find a way to enter the Grade 2 Dragon World.'