Transparent image prints as slightly shaded

I have a couple of images with transparent backgrounds. The transparent
portion overlaps a solid blue background border.

That overlap region looks OK in Publisher 2007 and in the PDF (on screen)
and if I print directly to the printer.

However, when I actually print the PDF on my color laser printer (HP 3600N)
that overlap (between background "blue border" and the transparent bit)
changes the color of the background border to a slightly darker shade.

Note:
When I print directly from the printer (Publisher 2007->HP printer) I have
to print RGB composite (b/c printer driver doesn't support CMYK)

When I print to PDF I print CMYK composite. Then when I print that PDF, I
think Acrobat Reader(8.1) is converting to RGB.


I'd love to chalk this up to printer wierdness but the printer isn't just
coincidentally printing wierdly at *just that area* ( a rectangle formed by
the intersection of the rectangular transparency and the background border).

It happens in two different places in my document with two different
transparent background images.

Any ideas why this might be happening?