Being an allergy sufferer, and possessing a medium complexion, I'm well acquainted with the under-eye concealers! In all honesty, I began using one not long after my first child was born. Oh, those long, often sleepless nights!
Anyway, I've dabbled with many types, and shades, of a variety of covers, and concealers, through the years, learning which ones did the most, with the least amount applied.
Try a concealer shade very close to the shade of foundation you are using. This allows for a nice blending, and you won't end up looking like you're wearing one of those little eye masks, in a color far lighter than the rest of your complexion!
If you have extremely dark circles, those dark brown ones, for women with medium to olive complexions, and those deep red ones, for fairer complexions, I recommend trying the yellow, or green concealers. They work amazingly well! Try applying a bit of the one that's right for you, then, just a touch of your regular cover, and blend gently. The yellow (for medium/olive complexions) or green (for fairer complexions) will actually do a bit of neutralizing of the brown, or red areas.