I've tried several different things and failed every attempt
The above is potentially a key to all of this - whose to say something that you tried was correct, but you implemented it wrong and therefore do not know it was the answer.
Most people go the route of 'picking' - whether it be creating a system of linepicking or by using Blitz3D's built-in 'Pickmode'.
You can set an entity (ie a mesh/piece of scenery) as being an 'obscurer' via EntityPickMode.
If you've not tried that then play around with it - other than that post a bit more info regarding what you have tried as the 'PickMode' solution is normally the first thing someone would try.