Monitor itself is probably the cause.
Internal problem with backlight dropping out.
The ultimate test is to take the monitor to a different computer or replace this monitor with a known good one and see if the problem still happens. They do die this way to work for a few seconds and drop out.
I had an old Dell monitor that did this and when I opened it the capacitors inside ruptured in a flat screen display manufactured in 2006. This monitor was manufactured during a capacitor plague where billions of capacitors with a stolen electrolyte recipe were made and the person who knew their recipe would be stolen sabotaged the recipe to be stolen is rumor has it as the cause. Pretty crafty to make a recipe that is limited life and passes component quality testing to make its way into products manufactured with these caps and then within about a year or two a flood of electronic device failures.
Monitors are cheap a new flat screen of a 20-24 inch can be purchased for around $100 USD, And at a second hand store a quarter of the cost new usually in the $25-$40 range. If its an older 14, 15, or 17" non-widescreen display I have seen them for as cheap as $10 each. I bought three 17" non-wide screen displays once for $30 at $10.