Converting .ai to pdf is changing colour of font

Hi all, 

I am creating a plan in illustrator and when saving the doc as a PDF it is changing the font colour from black to blue. I am using the same page for two different layers, and one layer saves to PDF perfect (stays black) and when I turn that layer off and turn the other on (same colour, same format) it is changing my font to blue when I save as PDF. 

Please help!! 

Thanks