The rules state that if both people want to reveal at the same time, the person with initiative gets to reveal first. Technically there is no "Declare Step" for revealing enchantments. The FAQ states you simply flip it over and it takes effect. However, there has to be some civilized method for which to determine if you both are wishing to reveal an enchantment. The most appropriate method is probably asking your opponent if they wish to reveal or pass priority to you, but many of my opponents do not do this and so instead I often interrupt them with the fact that I have initiative and wish to take advantage of my priority.
The main point though, is that your opponent does not get to see your Healing Charm before he reveals Poisoned Blood if he wishes to use the Poisoned Blood to stop the Healing Charm. He has to just guess and take a chance that that's what your enchantment is. Once Healing Charm has been revealed, it takes effect immediately.