Monitor Blanking does not consistently occur on XenDesktop 5.5 and 5.6 when connecting to a physical computer using HDX 3D Pro. This would result in the user sitting at the console of the VDA being able to see what is being done inside the ICA session.
This is a limitation in XenDesktop 5.5 and 5.6 and is a known issue.
Upgrade to XenDesktop 5.6 Feature Pack 1 and use the Remote PC feature, which has been tested with consistent monitor blanking.
Read the original post here.