When you cast the Healing Charm, he can choose to reveal the Poison Blood during the spell casting steps before learning what it is you have cast. They may also reveal Poison Blood after you have finished casting but before you declare a desire to reveal your spell, of course.
Then sometime after that you declare that you wish to reveal your enchantment. If your opponent has initiative, they may interrupt you by saying they also wish to reveal an enchantment before yours has been revealed. In which case, they may also reveal Poison Blood before learning that you have a Healing Charm in place. If they do, you do not have to reveal your Healing Charm. You may state that you've changed your mind and keep it face down.
Once you do reveal Healing Charm, if they have not already revealed Poison Blood, you immediately gain the benefit of your spell and they do not get a chance to respond to it before it takes effect.