Hi csk,
The iblck function should work in LabWindows/CVI. If you check out the NI-488.2 Help file by browsing to the Start Menu>>All Programs>>National Instruments>>NI-488.2>>NI-488.2 Help, you can find details on that function by searching for "iblck" from the search tab.
Try this out and see if it will work for you. Best of luck to you!
Chris R.
Applications Engineer
National Instruments