I can't give you specific guidance with regard to Libre Office, but I can tell you what the problem most likely is: your background is a simple black, where your PNG blacks are RGB Black. You'd think, intuitively, that these would be the same, but they're not. Black in RGB is what is termed a "Rich Black" or "Built Black" and actually consists of Black plus Cyan, Magenta and Yellow. Straight black is actually a very dark gray; it needs the other colors to absorb more light and become "more black."
Look in Libre Office for a setting that makes your entire document RGB, or a color that is Rich Black, and see if that helps. It might work to build your background using cyan, magenta and black objects one on top of another, but without knowing how Libre Office handles such things, I couldn't guarantee the result.