Answer updated based on comments
This is going to be a little more work than you probably wanted it to be. You will need a database with 2 tables. Table will will hold active sessions IDs and table 2 will hold messages:
tblSessions
   ID            Int
   LastSeen      DateTime
   SessionID     Varchar(255)
tblMessages
   ID            Int
   Timestamp     DateTime
   SessionID     Varchar(255)
   Message       Varchar(255)
When a visitor comes to your page, you need to check if the visitor has a session ID. If the visitor does have a session ID already, update the LastSeen column in the tblSessions table. If the visitor doesn't have a session ID, assign on and add it to the tblSessions table. This code should be run on all your pages when they are loaded.
You will need to run a query on the database table tblSessions to remove all entries that have a LastSeen older than some X time. The value of X should be say 5 min. This query could be run at the top of each page load, or in a server backend process.
Now, anytime you want to flash everyone's screens, you add an entry in tblMessages for each entry in tblSessions. Set the Timestamp to the time you send the message, and set Message to "flash".
On the browsers side in javascript, setup a polling function with setInterval. In your polling functions call an ajax script to a server side page to return any messages. This server side script should query the tblMessages for any entries for the current session ID and pass them back. It should also remove the entries from the table.
Back in your javascript polling function you can check for the "Flash" message and flash the screen. The more frequently the polling function is called the more realtime your visitor will be, but more of a load will be but on your server.
Just like with the tbleSessions table, you will want to remove old entries from the tblMessages table if they are over say X +1 min time or you will get old results in the table that can cause issues down the road.
So.. This will flash the screen for anyone visiting your page, at roughly the same time. I say roughly because there is no way to flash at exactly the same time with network lag and everyone polling at slightly different times.... Well no easy way at any rate.