I have this arrow decal, but it looks kind of soft and gray instead of dark and black (Like in material preview). I’s very hard to see in-game and I don’t know how to make it better 
Looking at your images, it seems like you are using very low settings to work in Unreal Engine. It looks like you are using the viewport setting Unlit. This will make things look washed out. Set the viewport mode to Lit instead, and the washed out decal problem should be resolved.