Perhapss blend modes are not used too often but for the sake of consistency these should be added....

Possible solution is to update the rna for the UI, updated code added below.

Also, makes use of defined constants instead of integers as used elsewhere.

From rna_lamp.c >>> rna_def_lamp_sky_settings

static EnumPropertyItem prop_blendmode_items[] = {
{MA_RAMP_BLEND, "MIX", 0, "Mix", ""},
{MA_RAMP_ADD, "ADD", 0, "Add", ""},
{MA_RAMP_MULT, "MULTIPLY", 0, "Multiply", ""},
{MA_RAMP_SUB, "SUBTRACT", 0, "Subtract", ""},
{MA_RAMP_SCREEN, "SCREEN", 0, "Screen", ""},
{MA_RAMP_DIV, "DIVIDE", 0, "Divide", ""},
{MA_RAMP_DIFF, "DIFFERENCE", 0, "Difference", ""},
{MA_RAMP_DARK, "DARKEN", 0, "Darken", ""},
{MA_RAMP_LIGHT, "LIGHTEN", 0, "Lighten", ""},
{MA_RAMP_OVERLAY, "OVERLAY", 0, "Overlay", ""},
{MA_RAMP_DODGE, "DODGE", 0, "Dodge", ""},
{MA_RAMP_BURN, "BURN", 0, "Burn", ""},
{MA_RAMP_HUE, "HUE", 0, "Hue", ""},
{MA_RAMP_SAT, "SATURATION", 0, "Saturation", ""},
{MA_RAMP_VAL, "VALUE", 0, "Value", ""},
{MA_RAMP_COLOR, "COLOR", 0, "Color", ""},
{MA_RAMP_SOFT, "SOFT_LIGHT", 0, "Soft Light", ""},
{MA_RAMP_LINEAR, "LINEAR_LIGHT", 0, "Linear Light", ""},
{0, NULL, 0, NULL, NULL}};