Make mesh transparent on event

I'm using the top-down template but with the camera angled slightly along y. I have buildings all over the map. When my character goes behind a building, I want that building to be transparent. I have an idea on how to check if my character is behind the building (probably a line trace from the camera to the building actor), but I don't know how to hide the building that is covering the character.

So, I have a BP with a mesh and a collision box around the mesh. When my character overlaps the collision box, I want the material of the mesh to become semi-transparent. I'm not sure where to get started with implementing this. I tried something called dynamic material instance but it didn't work in my case. Any suggestions?