I recently bought bought a studiofinish concealer , but it isn't really hiding all the darkness,only to some extent....and indeed the makeup artist who suggested me this color, said that concealers can only hide darkness upto a certain extent...they can't take away all the darkness and she claimed that it maybe the structure of my eye bones that making them look dark ...........but still I am confused that why do they say that it can even hide tatoos ???

I still bought the conealer and though that it would work when I'll try myself...but it is not hiding any darkness and when I put it, it seems that it is hiding but after 1 hour maybe , my darkness shows up again.........please tell me why it is happening ? and is there any SPECIAL way of applying the concealer ?
I am very depressed with MAC concealer....

Are you weating it with foundation? If so, try dabbing on a bit more foundation under the concealer. If you put your eye shadow on after foundation, some actually falls under your eyes, making them appear darker. Try putting on eyeshadow on before foundation and concealer.
Since you say that the concealer works for a bit then fades out, dust some loose mineral powder on top of it. You MUST wear powder on top of concealer or else the dark spots will show up again due to your pores sucking up the concealer. Put powder on before and after you use concealer.