2.1.21 [ISO32000-1] Section 11.3.5, Blend Mode
The specification states:
In principle, any function of the backdrop and source colours that yields another colour, Cr, for the result may be used as a blend function B (Cb, Cs), in the compositing formula to customize the blending operation. PDF defines a standard set of named blend functions, or blend modes, listed in Tables 136 and 137.
For all blend modes other than Normal, the shape parameter is ignored.