Kuro knew that if he doesn't open his seal he would ultimately lose against his opponent but if he or Shiro opens their seal then their body would suffer harm.
Their bodies were improvised by their sensei but this cause their body to go a drastic change and only the seal would be able to stop them from losing themselves.
For example he was compatible with fire element so his body was fused with a Legendary Grade dark and fire type monster to improve his strength.
He would become like a werewolf and because of the monster spirit in his body he would need to exercise caution if not he would go out of control.
But his opponent in front of him could take him even if his seal was opened heck he even had less than 50% chance of winning even if he opens his seal thankfully for him he was of the fire element if not he would surely lose.