Before Leo could start the evolution process, the Crimson Blood Dragon King promised to Leo in a serious tone which made Leo think, 'It's a pity I can't form a contract with it due to the grade difference.'
If Leo wanted to form a contract with the Blood Dragon after the evolution, he had to wait until one of his Tamer's Spaces upgraded to Diamond Grade.
Even now, Leo could not form a contract with it because he had already filled the three new beast slots in his Purple Grade Tamer's Space.
Initially, Leo thought that he could go back and upgrade his secondary Tamer's Space to Purple Grade and form the contract with the Crimson Blood Dragon. After that, he planned to evolve it so that he could have a Diamond Grade Beast before becoming a Diamond Grade Beast Tamer.
Nevertheless, after learning about Crimson Blood Dragon King's past, Leo decided to just help it remove the curse by evolving it.