Set the threshold for black mapping.
void pdfSetBlackThreshold(PDFHandle pdf, double t)
This function sets the black threshold.

The "black threshold" modifies the way the process color statistics are computed. If this is set to a non-zero value, any CMY values below the threshold will be ignored. This can be used, for example, to ignore minor color artifacts on color scans of black-and-white documents. Legal values are between 0 (don't ignore any CMY color) and 1 (ignore all CMY color). The default value is 0.

pdfSetBlackThreshold(pdf, 0.1);