In KDE, I think it is the default behavior (can be found System settings/in Desktop Effect - Dialog Parent effect) to darken the parent dialog window when there is an active dialog. It causes a problem when trying to adjust contrast or colors via a filter mask (since it creates a dialog, and the window goes dark, making it much more difficult to gauge contrast or colors). This problem can of course be solved by disabling the desktop effect, but it'll affect all windows, present and future. Is there any way for Krita to bypass this desktop effect? Of course this only affects Linux and not Windows - plus, I only use KDE 4 at the moment so it might be a KDE 4 specific problem.
Nope, Krita has no influence at all on this, we cannot do anything about it. While I appreciate that you took the time to report the issue, it's simply not something we can take action on. Sorry...