Quite a few lcd s have backlight bleed & it is really to the extent of this that would necessitate a return. Usually a bad bleed is over 10mm (1/2" nearly) usually from the top of the monitor. This, usually only shows up on a pure black screen & not on others. Do a search on monitor testing on this forum for good free testing programs & use them. Note: it is possible to get a monitor with zero bleed - I have one myself. If it overly annoys you either get a refund & try elsewhere or return to dell until you get a good one.

cheers