As a makeup artist one of this most common skin conditions I encounter is pigmentation. From sun spots to birthmarks I know how deep pigment can effect confidence in men and women.
My one tip- and it really goes against the grain here! - dark covers dark. Might sound odd but I have had countless clients, particularly with darker skin tones, who try to lighten the offending area, only to ultimately draw attention to it.
Use a solid cream concealer or foundation in the same colour as the pigmented mark and then follow up with your foundation that matches the rest of your skin. You'll be amazed by the results.
This tip works best on dark skin tones with dark pigmentation. If anyone would like tips on covering birthmarks of different tones (purple, blues etc) let me know!