There is a API to set how transparent objects are blended in WebViewer.
setTransparencyMode View — HOOPS Communicator documentation
TransparencyMode TransparencyMode — HOOPS Communicator documentation
1hwv.view.setTransparencyMode(Communicator.TransparencyMode.SingleLayer);
1hwv.view.setTransparencyMode(Communicator.TransparencyMode.Unsorted);