Translucent decal over emissive material is too translucent

I have a problem with some bloodspatter decals I’m working on, namely that when the decal is over an emissive material, it’s way too translucent (like barely a stain) where the effect I’m going for is to knock out the emissive completely. It looks fine on non emissive backgrounds (alpha is 100% white on the main puddles) but the light panels are totally killing the effect. Is this a known issue and is there something I can do to get the results I’m after?

I’ve attached a photo of the problem