A lot of people ask for this in one way or another and it seems at some point there was option for this but it was removed some time around when DBuffer got implemented.
The only Blend Mode for Decal material available is Translucent, the rest give errors and no one understands why those broken modes are even there. All decals in Content Examples are semi-transparent which shows that there’s only one blend mode available.
So I guess the question is: Is it something that’s not currently implemented, or not supported by current shader models, or just something that broke down and need to be fixed?