If potassium channels in a neuron were blocked, would it be possible to produce an action potential?

Assuming sodium channels are functioning normally, there would still be an action potential. Threshold would be unaffected, because it is a property of sodium channels. A peak voltage will be reached when the sodium channels become inactivated; this voltage may be higher than normal since usually there is a partial canceling of the rising voltage as potassium exits. Without a potassium current, the falling phase would be much slower, being dependent on the sodium-potassium pump removing the sodium ions that came in. The undershoot would be absent because it is a result of potassium current.