Alphablend

[DllImport("coredll.dll", SetLastError = true)]
public static extern bool AlphaBlend(IntPtr hdcDest,
   int xDest,
   int yDest,
   int cxDest,
   int cyDest,
   IntPtr hdcSrc,
   int xSrc,
   int ySrc,
   int cxSrc,
   int cySrc,
   BLENDFUNCTION blendFunction
);

Microsoft documentation: Link