Set the threshold for black mapping.
[read/write property] blackThreshold([out, retval] double *t) blackThreshold([in] double t)
This property contains 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.

pdf.blackThreshold = 0.1