The pointer is to a padded struct that matches the arguments to SET_PED_HEAD_BLEND_DATA(...). There are 4 bytes of padding after each field.
pass this struct in the second parameter
typedef struct
{
int shapeFirst, shapeSecond, shapeThird;
int skinFirst, skinSecond, skinThird;
float shapeMix, skinMix, thirdMix;
} headBlendData;