bitmap opacity

[AdriGerritsen]

My application uses a true color bitmap. However, some regions of the bitmap have to be made 'darker'. The same effect can be seen when using PaintShop's option "Selections, modify, overall opacity". My bitmap is drawn into a memory device that's copied to the actual screen. So far, all attempts to use BitBlt with several raster operations failed.
Who can give a clue??