I know other method for that, but actually it's no reasonable for use.
But yeah, u are right, there simply isnt any api that allows to replace a particular color (except black) with glass. So no luck.
DwmExtendFrameIntoClientArea and DwmEnableBlurBehind - one family, by meaning of DWM.
BTW, I am not using DwmEnableBlurBehind. I am extending the frame from the bottom of the window to get the "sheet-of-glass" kinda thing.