I think letterbox bars encoded into Blu-rays will usually be like RGB(16,16,16). So if there's video content under that it will show as darker than the black bars - unless you adjust the settings on the TV so that below 16 doesn't show.
Though really, if there is video content below RGB(16,16,16) and you adjust the TV controls (brightness etc.) so that video below that (the colour of the black bars) doesn't show and that the black bars are as black as possible, you also throw away part of the video picture. A better standard would probably have been for Blu-ray to know where the letterbox bars should be (eg. for 2.35:1 content) and automatically generate them in the Blu-ray player so they would always be the darkest part of the output image.