Normally I woudl send you to InDesignSecrets » Blog Archive » Eliminating YDB (Yucky Discolored Box) Syndrome, but in this case I think it may simply be that you are using the wrong transparency blend space.
Assuming that you not using PCL5C personality of the Phaser 7500 (which you really should effectively disable), the driver is effectively irrelevant to this situation since for PostScript printers, all PostScript is generated for InDesign, Illustrator, and Acrobat internally, not by system PostScript drivers.
Some thoughts on what to look at:
(1) What transparency blend space are you using? Specifying RGB for printing could be problematic.
(2) Do you get similar results if you export PDF from InDesign as PDF/X-4 and print from Acrobat Pro?
(3) There are some transparency effects that don't transfer well from Photoshop to InDesign via PSD. You might want to try saving as PDF from Photoshop and placing the resultant PDF in InDesign.
Thanks for all the replies. The InDesignSecrets was really helpful. I solved the issue by turning on "Simulate Overprint" in the Output dialog box. Haven't done extensive testing yet but so far, so good.
YDB: Never heard that one before.
Dov: Regarding the disabling of the PCL5C personality ... how would I go about that?
That should just be a matter of installing the printer withthe Postscript driver and not using an instance installed with the PCL driver if you have one.
Obviously, as Peter indicated, never install any PCL drivers. But also, from the control panel or more likely from the printer's website (yes, all Phaser printers actually have their own web server built in such that you can set some of the more obscure communications and paper-handling options), simply enable all the i/o ports that you use for PostScript only.