virtualbox.org

I am afraid you most likely will not find someone here that can help you with that on this forum (end-user orientated) :
As michaln pointed out, it is simply missing code in the implementation of Virtualbox. So unless someone provides this code, things won't just happen.

Regardless, if you want to ask directly to the Virtualbox Devs about this, I would suggest to use the vbox-dev mailing list, which is where you can directly contact them.
They might have some kind of answer for you.

As someone who has implemented flashing text for e.g. text terminal emulation, I can predict that this will most likely never happen.

In real old text terminals the flashing was handled by the hardware, it had no software cost. Nowadays the only way to flash text on a modern graphics display is to "manually" repaint it repeatedly. That involves a background task hanging off a timer wasting CPU scanning for the flash attribute. And constant repaints from inside a background thread is exactly the kind of thing that leads to display rendering issues. You don't accept those costs without an good reason - and one guy wanting a freebie isn't it. Come up with a big pile of cash and I'm sure it will be done. Otherwise forget it - learn to live without it, or do it yourself.

I think it's amusing that most emulators don't bother emulating a pretty basic feature of PC video hardware (eg, blinking cursors and blink attributes).

Because the 100% JavaScript IBM PC/XT emulator I wrote at jsmachines [dot] net emulates both.

Of course: it's trivially easy to do if you don't care about any of the side effects mentioned. I outlined how its done above, which is how I did it circa 1989.

Thanks for the blinks!! I don't know who added the code, but it really helps.

Thanks again
Marvin