Absolutely, and it is a fantastic sneaky play!
Casting a spell has 3 steps to it:
1) Cast Spell Step
2) Counter Spell Step
3) Resolve Spell Step
You can reveal an enchantment after any Step or Phase.
So, the opponent casts a Minor Heal spell. After the Cast Spell Step, reveal the Poisoned Blood. When the Minor Heal gets to the Resolve Spell Step, the target can no longer heal and the spell is discarded without effect! The opponent pays the full mana cost or Minor Heal and loses the spell and action!
