A stock Gloom Blade is a master-quality shortsword in bright light and a +2 master shortsword in dim or lesser light. Specific weapons can't be modified with property runes, but they can be further modified with potency runes.
So how does that apply to a Gloom Blade? Are they ignorant of each other, such that I have to start off with a +1 rune (making it +1 in bright light and +2 in dim light)? Do they stack, such that putting a +3 rune lets the weapon act like an early-access +5 weapon? Do they not stack, but I can still begin my improvement at +3 rather than +1?