However if I change the transperency blend space from CMYK to RGB
Your cyan blue color must be RGB right? The PDF/X standards include an Output Intent which is a CMYK profile, so when you open the PDF in AcrobatPro the default color simulation is a soft proof to the output intent and any out-of-gamut RGB colors would be brought into the simulation profile's color space. Your co-workers might be viewing in a recent version of Acrobat Reader, which doesn't simulate the output intent