Each Dark Pokémon has a Bar showing Darkness instead of Experience. This is set out in Five Bars. When you have 2 Bars Cleared, you get one of the Missing attacks other than Dark Rush, When 3 you get another, When 4 are gone you get the 3rd...however when 5 are gone...nothing

To heal the Pokémon you need to do the following:

• Keep it on your team
  
• Use it when Battling
  
• Call Out to it to get it out of 'Hyper State'
  
• Massage it with the Cologne Case
  
• Use Celebi to totally heal it (Can only be done twice in the entire game)

However once all 5 bars are empty, it is still a Dark Pokémon. How do you heal it without Celebi? here's a Step By Step guide.

Step 1: Go To The Celebi Shrine near Ageto Village	Step 2: Pick the Pokémon which has cleared it's Dark Meter (Hyper Pokémon cant be healed until they are no longer hyper)
Step 3: Your Pokémon's Darkness Will be gone	Step 4: Your Pokémon will now gain all the experience it got from battling whilst it was Dark
Step 5: If it goes by a certain level, you can evolve it or teach it new attacks, including the 4th one	Step 6: After this is done, you can now re-name your Pokémon

It is as simple as that, now the Pokémon is yours