Do you see it in Overprint Previe and with high quality image display in InDesign?
Apple Preview is not a good and reliable PDF viewer, don't use it. If something is shown wrong then in Preview.
Don't open any PDF which were not created with Illustrator in Illustrator, it will will destroy the fonts.
- What ius the transparency blending space of your document?
- Which black are you using? In  or a self defined?
- Are you using any transparency effects, like drop shadow or shine or …?
- Is the text above or below the image? Do you work with layers?
- How do you produce your pdf? Export? Printing? Which settings?