100 black becoming 4 colour black on export where a tint is used

Hi, I have a 2 page form which need to be black only. The ONLY colour used throughout it the 100 Black Swatch.

Page 2 uses 60% of this swatch. When I export the PDF to be print ready, the print checks show that the 60% Black, has now become CMYK... why is this? How can I amend it? 

Thanks!