Could anyone explain to me in simple terms how it's possible to have a blackout on Vested stock? I was under the impression that once I'm "vested" the stock is now mine and I own it like any normal stock. Are "vested" shares something different? Could I say for example sell my shares on Robinhood if the blackout wasn't in effect or are they held by some third party?
Vested shares are not special in and of themselves. However employees often have blackout periods for any kind of stock they hold - ESPP, RSU, Options, etc. In many cases even stock you purchased separately with your own after-tax money can be subject to a blackout (though the company would not have a mechanism to directly block your trades you could run the risk of SEC violations).
The blackouts apply to employees because they often have insider information, even if that information may be of minimal real value. The SEC has really clamped down on tech stock trading in the last couple of decades. Those of us who had stock/options in the late 90's dot-com boom likely participated in several stock trades that would be outright illegal today.