concealer

Is the concealer you want to use a liquid or a stick? If it is a liquid, use a concealer brush or the tip of the ring finger to lightly tap concealer onto the blemish/dark circle and then gently blend it into the surrounding area. Keep doing this until the spot is covered, but do not over do it. Next you can apply the foundation and set with powder, or just use powder. If it is a stick formula, follow the same steps, but you will have a little harder time blending, in my experience. You can also use your foundation as a liquid concealer.
You should go to a beauty counter and have the consultant help you select the best color match of concealer, foundation, and powder for your skin tone. Of course, you need to look at the makeup swatches on your face in natural and indoor lighting conditions to make sure it doesn't look too light, too dark, too orange, or too chalky. If you cannot see where the makeup is on your face, it is a good match.