Conformance recommendation S11.13 has been clarified as follows in XPS v0.95.

If any gradient stops use an sRGB or scRGB color specification or the consumer does not understand the PageBlendColorSpace PrintTicket setting, consumers SHOULD blend colors between gradient stops in the color space indicated by the ColorInterpolationMode attribute of the gradient brush. If none of the gradient stop elements uses an sRGB or scRGB color specification and the consumer understands the PageBlendColorSpace PrintTicket setting, the PageBlendColorSpace PrintTicket setting SHOULD be used. This SHOULD be a linear, channel-by-channel blend operation.