If you want to keep the key blocked, then don't call preventDefault(). The default action on a blocked key is to "block" it. By stopping the action you are allowing the key to proceed through without being blocked. Hope that makes sense.
I am running it of a Intel STB. What are the player keys equivalent in the simulator ?
Unfortunately the blockedKeys functionality hasn't been certified yet on the Intel platform, so I can't insure it's working 100% yet. To test in the WDK to see if that's the issue, the command to type in via the console is:
for play and:
for pause (stop follows a similar pattern).
Make sure you have switched to your widget context before issuing those commands.