Col, if you look at the histogram data you will note that the black point (far left) does not start at 0 (or close too). Instead it starts very high indicating more data could be stretched from the image or it has been clipped by another processing routine.
When working with levels and curves - always set your levels white point first, then start working with curves. Finally, don't make huge changes to your black point until close to the end of your processing routine.
The web has loads of information on histograms. Understanding how they work is critical if you want good results. They are also a good tool for evaluating your data. Here's one website of many available -
http://www.larry-bolch.com/histogram/ The first example show black clipping. Hope this helps.
