Query on DrvAlphaBlend and AlphaBlend

We are developing a fake graphics driver for which the primary requirement
is to transmit the pixel data over Ethernet which will displayed by a remote
monitor connected to a custom developed hardware. We have based our driver
on the mirror driver sample, and the driver seems to be working fine.

Added to the above requirement, we also have an additional requirement where
certain area’s of the GUI should be differentiated by the hardware so that
it can perform some additional actions for those areas. We were planning to
utilize the alpha channel in the frame buffer to achieve this. The plan was
that the GUI would set a specific alpha value by making use of the
AlphaBlend API, which would result in the frame buffer alpha channel to
contain the blended alpha value. (According to the formulas mentioned in the
DDK documentation of DrvAlphaBlend). But during the initial testing we have
noticed that AlphaBlend calls from the app does not really land up in the
DrvAlphaBlend call. The call is properly hooked, and I can see the call
landing up in DrvAlphaBlend if I try to move an ICON over the desktop, but
not when we make a call to AlphaBlend. We find calls to DrvBitBlt instead.
Is this the expected behavior? If it is, then is there a different way by
which the application can set the per pixel alpha which will get reflected
in the framebuffer?

Any clues / hints would be really helpful.

Thanks,
Anees