Radiator - Black Foam Spewing from Top Port

1999 Mystique, V6, 150,00 Miles. Driving 70, noticed what I thought was white smoke in mirror, immediately exited freeway when engine cut off, and realized it was steam from the under the hood. Once I opened the hood, what would be best described as steam and a black/grey foam was spewing from the left (driver) side port on top of the radiator. A little foam and a lot of steam for the next 15 minutes. Level & color in the coolant reservoir tank are the same as when check recently. Oil level / color are good. No other areas of leakage.
Thoughts / Suggestions on what might have caused this issue?
Are the 2 nozzles on top of radiator designed to be pressure relief valves? (only steam from diver side port).
I figure need to replace radiator, hoses, thermostat, water pump, transmission cooler lines.
Anything else I should replace, look at, or be concerned about? .