If you lessen the brightness then power off and power back on, it should stop then.
While I was in my B&N store their demo was doing that. I told sales girl she might want to put another on display because obviously this one has problems. I ended up buying an HD+ but not because of the glitch, just because I wanted the bigger screen.
I've had mine since the first weekend--this has not been an issue thus far.
I get it all the time coming out of the lock screen.
Open book with complex rendering. Sleep Nook. Press power again and slide to unlock. It will generally blink for a few seconds and then get on. Sometimes, it never comes back, but hitting power twice allows me to try again.
I have a single Nook HD and unfortunately it has this strange issue.
Sometimes, after 15 minutes of testing an app the screen will start blinking.
It's a regular cadence - as if it's done by the software on purpose.
Sometimes - It works to power screen off and then on.
Sometimes - It turns Nook HD screen off permanently and I have to restart.
I'm glad to know it's not an isolated incident as that increases the chance a software fix will be forthcoming.