For the background, typically, you record a background image that you then subtract from your test images (for dark background). In your case, however, you could simply threshold the image. Assuming that your image is uint8, does
yourimage(yourimage > 127) = 255;
work for you?
However, if you're going to binarise your image, then you don't need to adjust the contrast or play around with the background,
imbinarize(yourimage)
should isolate the black from the background.
