What I did
Opened up the sketchup drawing then opened up styles and edited the current style, setting background, sky and ground to white, then saved.
Opened up my layout and updated the reference links.
The imported drawing still showed the black blocks, so I right clicked on the drawing within the layout and selected styles.
this showed two style options one with  next to it.
I selected this style that DID NOT have a tick next to it and this seemed to resolve the issue.
I had several instances of the same sketchup drawing on the layout and had to do it for each instance.
Some times I had to click back and forth between the two styles before it would work.
Im running sketchup 2017 on a new macbook pro with serria os x.
I think it could be due to transparent backgrounds, I moved a series of jobs over from my old macbook pro running 2015, where everything displayed fine and these had transparent backgrounds as I often overlay the imported drawing.