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I am driving a 2017 Honda Ridgline with cruise control, and lane keep assist engaged. Traveling at the legal speed of 70MPH (112KPH) suddenly I become unconscious.

If my hands and body don't alter the steering, the vehicle will continue down the road for sometime. I am not sure exactly how the vehicle will handle the loss of the driver.

In the old days you could, reach over and turn off the key. The vehicle would slow and stop. In new vehicles there is a push button. Powering down the vehicle in motion, may or may not work, and if it does you would probably lose the lane keep assist and automatic braking.

What is the safest course of action for a totally blind passenger, who is suddenly faced with stopping a vehicle from the passenger seat?

NOTE: In this vehicle the parking brake is in the drivers pedal area, the shifter is located in the center console, the steering wheel has lots of buttons (the buttons don't have braille)

This answer offers an addition to the fine answer of @Zeiss Ikon, but does not repeat any of his advice.

You want to signal to other cars, especially a police car, that you have a problem.

Take a few minutes to acquaint your blind passenger with the position of the button for the hazard lights. In my car, there is a large button within easy reach of the passenger. The passenger should also know how to open the window on his side, so he can yell a warning and for help.

At a minimum, several police cars can escort your car and isolate your car from traffic, meanwhile trying to talk your passenger into effective action, such as lifting your foot from the accelerator. This action also may be something that you can rehearse with your passenger before you start off, in Park, of course. (But I don't know if this can be done while the passenger is belted in. Will try to simulate later.)

As for the comment that you should not drive if you have a condition that may cause unconsciousness, fine, if you know it. But heart attacks and strokes can happen with no prior warning.

Addition: This is not a scenario that a sighted person normally thinks about. And I don't like telling an adult what he "ought to do". But a blind person needs to think through a lot of what do I do if.... scenarios, including this one.

Based on the latest comments, lane assist will only work for a minute or so without hands on the wheel. That means you need to accept that if the driver is grossly incapacitated an the passenger isn't capable of taking over, at least to the level of shutting off the engine and steering to the shoulder while the vehicle coasts to a stop, the same thing will happen as would in a 1950s vintage car: it will wander into adjoining lanes or completely off the road, and will crash into something.

The only real good news here is that a passenger in a modern car is much better protected in any kind of crash that would have been the case half a century ago. Crush zones, air bags for side impact, not just front end crashes, and so forth. A crash can still kill, but the odds are far better than they'd have been when I learned to drive in the 1970s.