What is the perfect concealer?

For under eye darkness, I use Benefit's Erase Paste. You can get it online from Benefit or Sephora, or go to a Sephora. It isn't cheap by any means, $26 for a jar. However, it lasts FOREVER because you only need the tiniest amount to get good coverage. I have a miniature jar of it and after almost a year, it's no where near even half gone. It has a peachy undertone to help counteract the purple and blue hues under your eyes. If you can't afford this, just make sure to look for a concealer that has a peach or salmon-pink undertone to it. And be careful choosing a concealer that is lighter than your skin. You can go about 1 shade lighter under your eyes, but any lighter with over-highlight the area and will look very strange. If you apply concealer that is any lighter than your skin to regular blemishes, it will highlight and therefore bring attention to them, rather than making them disappear. Good luck!