It may be confusing at first, but It's important to note that it doesn't matter if the model is female or male, faceblend allows any of these faces ID to be merged together, and each of them have their own skin tone.
Shape / Skin First ID: "Mother"
Shape / Skin Second ID: "Father"
Again all faces belong to any gender, so technically speaking Mother + Mother works. You can create characters that are not possible in GTAO since Men / Woman are separated in UIs.
Shape / Skin Mix: Float values ranging from 0.0f ("Mother") to 1.0f ("Father").
Third ID: If you feel like another ID should be mixed in. Personally I leave 3rd IDs to 0 and 0.0f mix. Basically they appear to relate to the parent / legacy character customization in GTAO.
IsParent: I'm assuming SET_PED_HEAD_BLEND_DATA can be called again to reference this character as a parent. Or if you like headaches.
SET_PED_HEAD_BLEND_DATA(Ped ped, int shapeFirstID, int shapeSecondID, int shapeThirdID, int skinFirstID, int skinSecondID, int skinThirdID, float shapeMix, float skinMix, float thirdMix, BOOL isParent)
Here's all the UNTOUCHED faces ID's for Freemode characters.